Incorporation Into Select Uranyl Phases and Thermal

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Np5+ INCORPORATION INTO SELECT URANYL PHASES AND THERMAL

ANALYSIS OF SELECT URANYL PHASES

A Dissertation

Submitted to the Graduate School

of the University of Notre Dame

in Partial Fulfillment of the Requirements

for the Degree of

Doctor of Philosophy

Amanda Leigh Klingensmith

Peter C. Burns, Director

Graduate Program in Civil Engineering and Geological Sciences

Notre Dame, Indiana

June 2008

Amanda Leigh Klingensmith

Np5+ INCORPORATION INTO SELECT URANYL PHASES AND THERMAL

ANALYSIS OF SELECT URANYL PHASES

Abstract

Amanda Leigh Klingensmith

Alteration of spent nuclear fuel in a geological repository under oxidizing conditions is likely to result in abundant uranyl compounds. The proposed repository at

Yucca Mountain, Nevada is intended to store about 70,000 metric tons of spent nuclear fuel in the unsaturated zone of a welded tuff sequence. Following failure of canisters that encapsulate the waste, contents may be exposed both to air and water and undergo repetitive wetting and drying events. Incorporation of radionuclides into the uranyl alteration phases may significantly reduce their mobility, thereby impacting repository performance. Of particular interest is 237Np owing to its long half-life (2.14 x 106 years) and potential mobility in groundwater.

Powders of the synthetic uranyl phase soddyite, (UO2)2(SiO4)(H2O)2, a framework type structure, and uranophane, Ca[(UO2)(SiO3OH)]2(H2O)5, kasolite,

Pb[(UO2)(SiO4)]H2O, Na compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7, and becquerelite,

Ca[(UO2)3O2(OH)3]2(H2O)8, all of which are sheet type structures, were synthesized in the presence of Np5+ under varying temperature and pH conditions. Uranophane, kasolite, boltwoodite K[(UO2)(SiO3OH)](H2O)1.5, and Na boltwoodite

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Amanda Leigh Klingensmith

K,Na[(UO2)(SiO3OH)](H2O)1.5 were synthesized in the presence of Np as well as P, Ca and/or Mg. Single crystals of Na metaschoepite, Na[(UO2)4O2(OH)5]·5H2O were synthesized in the presence of Np5+ and laser ablation verified that Np can be incorporated within the structure of a uranyl phase.

Incorporation of Np5+ into soddyite increased steadily with synthesis temperature.

Np incorporation into uranophane, becquerelite, and kasolite was not dependent on synthesis temperature. Np uptake in uranophane and kasolite was found to be dependent on synthesis pH, with an increase in Np uptake with higher pH. Uranophane, boltwoodite and Na boltwoodite showed an increase in Np incorporation in the presence of P. Boltwoodite showed an even higher Np uptake when Mg and P were both present in the synthesis.

Thermal analysis was completed for the uranyl phases soddyite, becquerelite, Na compreignacite, uranophane, and kasolite. TGA curves for becquerelite, Na compreignacite and uranophane showed loss of interlayer water groups by 100°C.

Soddyite and kasolite showed more gradual TGA curves and retention of water groups up to 400°C for soddyite and 550°C for kasolite, with agreement shown by high temperature powder XRD data.

CONTENTS

FIGURES ...... vi

TABLES ...... xvi

ACKNOWLEDGMENTS ...... xvii

CHAPTER 1 INTRODUCTION ...... 1 1.1 Uranium and the environment ...... 1 1.2 Crystal chemistry of uranyl phases ...... 4 1.3 Natural analogues of nuclear waste ...... 7 1.4 Uranium mineralogy and nuclear waste ...... 9 1.5 Np5+ crystal chemistry ...... 12 1.6 Thermal stability of U6+ phases ...... 17 1.7 Hypotheses ...... 21

CHAPTER 2 MATERIALS AND METHODS ...... 22 2.1 X-ray powder diffraction ...... 22 2.2 Single crystal X-ray diffraction ...... 23 2.3 Inductively coupled plasma-mass spectrometry: ICP-MS ...... 23 2.4 Inductively coupled plasma atomic emission spectrometry: ICP-AES ...... 25 2.5 Thermogravimetric analysis ...... 26 2.6 High temperature stage with powder X-ray diffraction ...... 27 2.7 pH measurements ...... 27 2.8 Laser-ablation inductively coupled plasma mass spectrometry ...... 27 2.9 Electron microscopy and microprobe...... 28 2.10 Ultraviolet visible spectroscopy ...... 28

CHAPTER 3 CRYSTAL STRUCTURES ...... 30 3.1 Uranophane sheet topology ...... 30 3.1.1 Uranophane ...... 31 3.1.2 Boltwoodite and Na-boltwoodite ...... 32 3.1.3 Kasolite ...... 33 3.2 Soddyite ...... 33 3.3 Becquerelite ...... 37 3.4 Na compreignacite ...... 38 3.5 Na-substituted meta-schoepite ...... 38

CHAPTER 4 SYNTHESIS OF URANYL PHASES ...... 40 4.1 Synthesis of uranophane ...... 41 4.2 Synthesis of soddyite ...... 41 4.3 Synthesis of becquerelite ...... 42 4.4 Synthesis of Na compreignacite ...... 42 4.5 Synthesis of kasolite ...... 42 4.6 Synthesis of boltwoodite ...... 43 4.7 Synthesis of Na-substituted boltwoodite ...... 43 4.8 Synthesis of Na-substituted meta-schoepite ...... 43

CHAPTER 5 Np5+ INCORPORATION IN Na SUBSTITUTED METASCHOEPITE .. 45 5.1 Crystal synthesis ...... 45 5.1.1 Synthesis of Na-substituted metaschoepite ...... 45 5.1.2 Natural specimen ...... 46 5.2 Chemical analysis ...... 47 5.3 Structure solution ...... 50 5.3.1 X-ray data collection ...... 50 5.3.2 Structure refinement...... 51 5.3.3 Structure description ...... 59 5.4 Np5+ incorporation into Na-MS-CRY-Np ...... 66 5.5 Discussion ...... 68

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CHAPTER 6 NEPTUNIUM SUBSTITUTION IN SYNTHETIC URANYL PHASES AS A FUNCTION OF TEMPERATURE AND pH...... 70 6.1 Synthesis ...... 71 6.2 Analysis of samples at ANL ...... 71 6.3 Analysis of samples synthesized at the University of Notre Dame ...... 74 6.4 Results ...... 75 6.4.1 Results for soddyite...... 77

6.4.2 Results for uranophane ...... 83 6.4.3 Results for Na compreignacite ...... 87 6.4.4 Results for becquerelite...... 90 6.4.5 Results for kasolite ...... 93 6.4.5.1 Np incorporation in kasolite as a function of pH ...... 93 6.4.5.2 Results for Np incorporation into kasolite synthesized at different temperatures ...... 94 6.5 Discussion ...... 94

CHAPTER 7 Np, P, Mg, Ca, INCORPORATION INTO URANYL PHASES ...... 99 7.1 Experimental ...... 99 7.1.1 Synthesis ...... 99 7.1.2 Chemical Analysis ...... 100 7.2 Results ...... 102 7.2.1 Results for uranophane ...... 102 7.2.2 Results for kasolite synthesized in the presence of P ...... 104 7.2.3 Results for Na boltwoodite synthesized in the presence of P ...... 105 7.2.4 Results for boltwoodite ...... 107 7.3 Discussion ...... 108

CHAPTER 8 THERMOGRAVIMETRIC ANALYSIS AND THERMAL POWDER X- RAY DIFFRACTION OF SELECT URANYL PHASES ...... 111 8.1 Experimental Procedures ...... 113 8.2 Unit cell calculations ...... 119 8.2.1 Internal standard...... 119 8.3 Results for becquerelite ...... 122

8.3.1 Thermogravimetric analysis for synthetic becquerelite ...... 122 8.3.2 High temperature powder X-ray diffraction of synthetic becquerelite ...... 124 8.4 Results for uranophane ...... 135 8.4.1 Thermogravimetric analysis for synthetic uranophane ...... 135 8.4.2 High temperature powder X-ray diffraction of synthetic uranophane ...... 137 8.4.3 Uranophane rehydration...... 149 8.5 Results for soddyite ...... 152 8.5.1 Thermogravimetric analysis for synthetic soddyite ...... 152 8.5.2 High temperature powder X-ray diffraction of synthetic soddyite ...... 154 8.6 Results for kasolite ...... 163 8.6.1 Thermogravimetric analysis for synthetic kasolite ...... 163 8.6.2 High temperature powder X-ray diffraction of synthetic kasolite ...... 165 8.7 Results for Na compreignacite ...... 172 8.7.1 Thermogravimetric analysis for synthetic soddyite ...... 172 8.7.2 High temperature powder X-ray diffraction of synthetic Na compreignacite ....174 8.8 Discussion ...... 183

CHAPTER 9 CONCLUSIONS AND OUTLOOK ...... 186

APPENDIX ...... 190

REFERENCES ...... 205

FIGURES

Figure 1.1 Eh-pH diagram for U under synthetic J-13 water conditions at 25°C. The diagram shows the stability fields of predominant aqueous U species (Johnson and Werme 1994)………………………………………………………………... 3

Figure 1.2 A) shows the typical uranyl UO2 ion with an average bond length of ~1.79Å to the apical O. B) shows the further coordination by four, five and six equatorial ligands to create square, pentagonal, and hexagonal bipyramids with average bond lengths of ~2.28Å, 2.37Å, and 2.47Å respectively. (Burns, Miller et al. 1996)… 6

Figure 1.3 (Crowley 1997) “Toxicity of nuclides in spent fuel from a light water reactor, shown for isotopes of americium, cesium, lead, neptunium, plutonium, proctectinium, radium, strontium, technetium, thorium, and uranium. Toxicity is defined here as the volume of water required to dilute the radionuclide to its maximum permissible concentration per unit mass of the radionuclide. After a few hundred years toxicity is dominated by the actinides (U, Np, Pu), their progeny (such as Ra and Th) and certain fission products. The toxicity levels in this diagram are for direct human ingestion of spent fuel and would not necessarily apply for other exposure pathways. For example, radionuclide toxicities for exposures from groundwater would be dominated by isotopes that are soluble and not sorbed completely by the host rock; for Yucca Mountain, such isotopes are believed to include 99Tc, 129I and 237Np”…………………………. 13

+1 Figure 1.4 (A) The (NpO2) with an average bond length of ~1.84 Å. (B) shows the +1 (NpO2) further coordinated by four, five and six equatorial ligands to create square, pentagonal, and hexagonal bipyramids………………………………… 15

Figure 3.1 The uranophane-type sheet showing the linking chains of yellow uranyl pentagonal bipyrimids and turquoise silicate tetrahedra……………………….. 31

Figure 3.2 The structure of uranophane, Ca[(UO2)(SiO3OH)]2·5H2O The top figure

shows the linking chains of yellow uranyl pentagonal bipyrimids and turquoise silicate tetrahedra to form the uranophane-type sheet. The bottom figure shows

the interlayer Ca and H2O groups sandwiched between the sheets……………. 32

Figure 3.3 Boltwoodite structure. The top left image shows a side view of the sheet structure through the interlayer. The top right shows a planar view of the uranophane type sheet in boltwoodite and the bottom image shows the alternating silicate tetrahedra……………………………………………………………… 34

Figure 3.4 A.) The typical uranophane type sheet for kasolite. B.) A side view of the sheet structure showing the interlayer of Pb shown as the dark blue spheres and its corresponding connectivity to the yellow uranyl bipyramid and turquoise silicate tetrahedra and one water group shown as the grey sphere. C.) Another side view of the kasolite structure with the unit cells shown as the blue dashed lines…… 35

Figure 3.5 A.) The structure of soddyite (UO2)2(SiO4)(H2O)2, the silicate tetrahedra are shown by the fuchsia tetrahedra, the uranyl pentagonal bipyramids are shown in yellow the unit cell is shown as the blue dashed lines. B.) Another angle on the soddyite structure showing the open channels along the X axis. C.) Another view showing the three dimensional framework of soddyite……………………….. 36

Figure 3.6 A. )A planar view of the becquerelite Ca[(UO2)3O2(OH)3]2(H2O)8, sheet B.) A side view showing the interlayer water as grey spheres and Ca2+ as blue spheres to be seen………………………………………………………………………….. 37

Figure 3.7 A.) Na-compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7, a planar view of the sheet of uranyl pentagonal bipyramids. B.) The interlayer connections through +1 the Na atoms shown as the black and white checkered spheres and H2O groups shows as the white spheres……………………………………………………... 38

Figure 3.8. Na-meta-schoepite, Na[(UO2)4O2(OH)5]·5H2O. A.) The uranyl pentagonal bipyrimids forming the sheet layer. B.) The bottom figure shows the interlayer +1 Na and H2O groups in red……………………………………………………. 39

Figure 5.1 SEM image of Na-MS crystal………………………………………………. 49

Figure 5.2 LA-ICP-MS data for a crystal of Na metashoepite. The crystal on the top left shows the ablation tracks of a 15µm laser……………………………………… 50

Figure 5.3 Polyhedral representation of the structure of Na substituted metaschoepite………………………………………………………………….. 62

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Figure 5.4 The sheet anion-topologies (Burns et al. 1996) corresponding to the sheets in Na-MS-CRY, metaschoepite and fourmarierite, with the locations of OH groups indicated by circles. Vertices in the anion topology that do not contain OH correspond to O atoms………………………………………………………… 63

Figure 5.5 Bond valence sums at O11 for all Na metaschoepite structures against corresponding b dimensions…………………………………………………… 65

Figure 6.1 Burns et al. (2004) results for Np incorporation into uranophane, Na

compreignacite, beta UO2(OH)2, and metaschoepite, with Np ppm of the total actinides on the Y axis and the initial quantity of Np5+ in the mother solution on the X axis..……………………………………………………………………… 76

Figure 6.2 Results of analyses for powders of soddyite versus synthesis temperature. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution……………………. 80

Figure 6.3 A.) Calculated solubility curves for Np2O5, NpO2(OH) and

NaNpO2CO3·3.5H2O in J-13 water at 25˚C (discussed in Chapter 1). B.)

Calculated solubility curves for Np2O5, NpO2(OH) and NaNpO2CO3·3.5H2O in UE-25#1(carbonate rich ground water from the Yucca Mountain site) water at

25˚C. The dashed lines indicate the solubility of Np2O5 using the value from Lemire (1984). (from Kaszuba and Runde 1999)……………………………… 81

Figure 6.4 The structure of soddyite structure showing the open cavities that form channels.……………………………………………………………………….. 83

Figure 6.5 Results of analyses for powders of uranophane synthesized from a solution with an initial pH of 5 versus synthesis temperature. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution……………………………………………. 84

Figure 6.6 Results of analyses for powders of uranophane synthesized from a solution with an initial pH of 4 versus synthesis temperature. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution……………………………………………. 85

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Figure 6.7 Results of analyses for powders of uranophane synthesized from a solution with an initial pH of 6 versus synthesis temperature. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution……………………………………………. 86

Figure 6.8 Results for powders of uranophane synthesized at 100˚C with an initial mother solution pH of 4.0, 5.0, and 6.0. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution………………………………………………………………….. 87

Figure 6.9 Results of analyses for powders of Na compreignacite synthesized at 100˚C versus pH. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using 0.5 M acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution…………………………………………………………………………. 88

Figure 6.10 Chemical analysis of Na compreignacite synthesized at 120˚C at different pH values of the initial solution. Only water-washed samples are shown as not enough powder was recovered for the acid-washed samples. The errors shown here are a standard 10% given by ANL………………………………………... 89

Figure 6.11. Analyses of powders of Na compreignacite synthesized with an initial synthesis pH of 5.5, at 100˚C and 120˚C………………………………………. 90

Figure 6.12 Results of analyses for powders of becquerelite synthesized at a pH 5 over the temperature range of 80-150˚C. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using 0.5 M acetic acid………………………………………………………………. 91

Figure 6.13 Results of analyses for powders of kasolite synthesized at 150˚C versus initial solution pH. Diamonds represent data for samples washed with hot water, whereas squares are for powders that were also washed using acetic acid. All samples were analyzed seven times…………………………………………… 95

Figure 6.14 Results of analyses for powders of kasolite synthesized at an initial solution pH of 5 over the temperature range 80˚C – 150˚C. Diamonds represent data for samples washed with hot water, whereas squares are for powders that were also washed using 0.5 M acetic acid. All samples were analyzed three times…….. 96

Figure 6.15 Powder X-ray diffraction patterns for kasolite synthesized with Np at 80˚C, 100˚C, 120˚C, and 150˚C………………………………………………………. 98

Figure 7.1 Analyses results for powders of uranophane synthesized without added P shown as the green square and red triangle (the red triangle is behind the green square), with the uranophane data from Chapter 6 also synthesized at a pH of 5.0……………………………………………………………………………… 103

Figure 7.2 Analyses results for powders of uranophane, the control experiment without P in the initial solution is shown by blue and green diamonds and the P experiment, with Np shown by purple and red squares…………………………………….. 104

Figure 7.3 Chemical analysis results for powders of kasolite, analyzed in triplicate. The control experiment without P in the synthesis is shown on the left represented at Np, the syntheses that included both Np and P are represented as P. The analyses of the acid washed samples are shown as red squares, the analyses of the water- washed samples are shown as blue diamonds.……………………………….. 105

Figure 7.4 Results of analyses for powders of Na boltwoodite synthesized in the presence of Np are shown on the left and powder of Na boltwoodite synthesized in the presence of Np and P are shown on the right. All samples were analyzed in duplicate. Analyses for the water washed samples are shown as red squares and acid washed samples are shown as blue diamonds…………………………… 106

Figure 7.5 Analyses results for powders of boltwoodite showing all four syntheses, the control experiment with only Np, the P experiment, the Mg experiment and the Ca experiment. All water washed samples are shown in red squares and all acid washed samples are shown in blue diamonds. Every sample was run in duplicate, certain samples are so close as to appear as one……………………………… 108

Figure 8.1 Thermogravimetric analysis curves for synthetic powders of kasolite, Na compreignacite, uranophane, soddyite, and becquerelite…………………….. 115

Figure 8.2 Thermogravimetric analysis segments for synthetic powders of uranophane held at 200˚C, 500˚C, and 900˚C, showing in dashed lines the temperature and time held, opposite the weight in percent lost in solid matching colored lines...... 116

Figure 8.3 A second thermogravimetric analysis curve for synthetic powders of uranophane, to examine the effect of holding at a temperature. The temperature was held at 75˚C, 200˚C, 450˚C, 550˚C and 900˚C. The dashed lines indicating heating trend, slopes for heating, horizontal for holding at temperature, with matching colored lines to indicate corresponding weight loss. The slight lipping on each held section is due to the time it takes for the entire sample to reach temperature, water loss will continue until all the grains have reached the holding temperature…………………………………………………………………… 117

Figure 8.4 Powder X-ray diffraction patterns taken for uranophane at 350˚C, while the specimen was held at temperature over the course of 12 hours………………. 118

Figure 8.5 Powder X-ray diffraction pattern for the Si internal standard shown with an overlay in blue of the matching ICDD pattern 27-1402, with the filament Pt peaks shown in red, corresponding to the matching ICDD peaks for pattern 04- 0802…………………………………………………………………………… 121

Figure 8.6 Thermogravimetric analysis curve for becquerelite from 30˚C to 700˚C…. 123

Figure 8.7 Powder X-ray diffraction pattern for becquerelite at room temperature with an overlay of the ICDD pattern for becquerelite…………………………………. 125

Figure 8.8 Graphical representation for the a, b, and c unit-cell dimension for becquerelite versus temperature………………………………………………. 127

Figure 8.9 Powder X-ray diffraction patterns with the corresponding temperature listed along the Z axis for synthetic becquerelite as the starting material………….... 128

Figure 8.10 Powder X-ray diffraction patterns for synthetic becquerelite at 30˚C-100˚C at 25˚C intervals…………………………………………………………………. 129

Figure 8.11 Powder X-ray diffraction patterns for starting material of synthetic becquerelite at 100˚C-300˚C at 25˚C intervals……………………………….. 130

Figure 8.12 Powder X-ray diffraction patterns for powders identified as becquerelite prior to heating, at 275˚C with an overlay of the peak positions for U3O8……….. 131

Figure 8.13 Powder X-ray diffraction patterns for powders identified as becquerelite at room temperature, over the temperature range of 300˚C to 500˚C at 25˚C intervals……………………………………………………………………….. 132

Figure 8.14 Powder X-ray diffraction patterns for powders of synthetic becquerelite identified at room temperature, at 500 - 800˚C, at 25˚C intervals……………. 133

Figure 8.15 Powder X-ray diffraction pattern for powders identified as synthetic becquerelite at room temperature, at 800˚C with an overlay of the peak positions for UO2 and U3O8, as well as Pt and Si peaks………………………………………………………………………….. 134

Figure 8.16 Thermogravimetric analysis curve for synthetic powders of uranophane from 30-700˚C……………………………………………………………………… 136

Figure 8.17 Powder X-ray diffraction of powders of synthetic uranophane at 30˚C with an overlay of the ICDD pattern corresponding to uranophane………………….. 139

Figure 8.18 Powder X-ray diffraction patterns for powders identified as synthetic uranophane at room temperature, from the range 30˚C to 500˚C at 25˚C intervals...... 140

Figure 8.19 Graphical representation of a, b, and c, unit-cell dimensions of the uranophane cell versus temperature…………………………………………… 142

Figure 8.20 Powder X-ray diffraction of powders of powders initially identified as synthetic uranophane from 30˚C to 125˚C showing a clear match for uranophane until 125˚C where broadening and peak loss can be seen……………………. 143

Figure 8.21 Powder X-ray diffraction of powders identified as synthetic uranophane at room temperature, showing overlays for the temperature range from 125 - 300˚C at 25˚C intervals………………………………………………………………. 144

Figure 8.22 Powder X-ray diffraction patterns for powders initially identified as synthetic uranophane for the temperature range of 300˚C to 500˚C……………………. 145

Figure 8.23 Powder X-ray diffraction patterns of powders initially composed of synthetic uranophane for the range of 500 - 675˚C at 25˚C intervals…………………… 146

Figure 8.24 Powder X-ray diffraction patterns for powders initially identified as synthetic uranophane from 700 - 900˚C at 25°C intervals……………………………… 147

Figure 8.25 Powder X-ray diffraction of powders initially identified as synthetic uranophane at 900˚C with an overlay of peak positions for Si, Pt, UO2, and CaUO3………………………………………………………………………… 148

Figure 8.26 Synthetic powders initially identified as uranophane heated to 300˚C, then cooled and rehydrated, showing a partial but incomplete recrystallization of uranophane……………………………………………………………………. 150

Figure 8.27 Synthetic powders initially identified as uranophane heated to 275˚C, then cooled and rehydrated, showing a recrystallization of uranophane…………… 151

Figure 8.28 Thermogravimetric analysis curve for powder of synthetic soddyite from 30 - 700˚C…………………………………………………………………………. 153

Figure 8.29 Powder X-ray diffraction pattern for synthetic soddyite at room temperature with an overlay of the peak positions in the ICDD pattern for soddyite…….. 155

Figure 8.30 Graphical representation of a, b, and c unit-cell dimension for synthetic soddyite versus temperature…………………………………………………… 156

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Figure 8.31 X-ray diffraction patterns for powder initially identified as synthetic soddyite over the temperature range of 30 - 175˚C at 25˚C intervals………………….. 158

Figure 8.32 Powder X-ray diffraction patterns for powder initially identified as synthetic soddyite over the temperature range of 200 - 400˚C at 25˚C intervals………. 159

Figure 8.33 Powder X-ray diffraction patterns for powder initially identified as synthetic soddyite over the temperature range of 300 - 500˚C at 25˚C intervals……….. 160

Figure 8.34 Powder X-ray diffraction patterns for powder initially identified as synthetic soddyite collected over the temperature range of 500˚C – 900˚C at 25˚C intervals……………………………………………………………………….. 161

Figure 8.35 Powder X-ray diffraction pattern for powder initially identified as synthetic soddyite powder collected at 800˚C with overlays of peak positions for Si and Pt, with partial matches to U3O8 shown as the orange lines that are unlabeled…. 162

Figure 8.36 Thermogravimetric analysis curve for synthetic kasolite from 30˚C - 900˚C………………………………………………………………………..... 164

Figure 8.37 Powder X-ray diffraction pattern for synthetic kasolite at 30˚C with an overlay of peak positions from the ICDD database matching kasolite, peaks that do not correspond to kasolite are Si peaks from the internal standard……….. 166

Figure 8.38 Powder X-ray diffraction patterns for powder initially identified as synthetic kasolite over the range of 30-800˚C at 50˚C intervals, 800-1000˚C at 25˚C intervals……………………………………………………………………….. 167

Figure 8.39 Unit cell dimension and graphical representation of the a, b, and c unit-cell dimensions for kasolite versus temperature………………………………….. 169

Figure 8.40 Powder X-ray diffraction patterns for powder initially identified as synthetic kasolite for the temperatures range of 550 - 800˚C………………………….. 170

Figure 8.41 Powder X-ray diffraction patterns for powder initially identified as synthetic kasolite at 1,000˚C with overlays of peak positions corresponding to Si, Pt, PbO, and U3O8…………………………………………………………………….. 171

Figure 8.42 Thermogravimetric analysis curve for synthetic sodium compreignacite from 30 - 900˚C……………………………………………………………………. 173

Figure 8.43 Powder X-ray diffraction pattern for Na compreignacite at room temperature with an overlay of the peak positions from the ICDD pattern for Na compreignacite……………………………………………………………….. 175

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Figure 8.44 Powder X-ray diffraction patterns of powder initially identified as synthetic Na compreignacite for the temperature range 30 - 500˚C……………………. 176

Figure 8.45 Powder X-ray diffraction patterns for powder initially identified as synthetic Na compreignacite over the temperature range of 30 - 225˚C at 25˚C intervals……………………………………………………………………….. 177

Figure 8.46 Powder X-ray diffraction patterns for powder initially identified as synthetic Na compreignacite over the temperature range of 200 - 350˚C………………. 178

Figure 8.47 Powder X-ray diffraction patterns for powder initially identified as synthetic Na compreignacite over the temperature range of 300 - 500˚C………………. 179

Figure 8.48 Powder X-ray diffraction patterns for powder initially identified as synthetic Na compreignacite over the range of 500 to 800˚C at 25˚C intervals……….. 180

Figure 8.49 Powder X-ray diffraction pattern of powder initially identified as synthetic Na compreignacite at 800˚C with an overlay of peaks corresponding to U3O8 shown as the blue lines………………………………………………………. 181

Figure 8.50 Powder X-ray diffraction patterns for powder initially identified as synthetic Na compreignacite at 800˚C with an overlay of peaks for U3O8 shown in the turquoise lines and Na0.35UO2.95 shown as the black lines.…………………… 182

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TABLES

Table 5.1 Crystallographic and refinement information for crystals of Na-MS ...... 48

Table 5.2 Atomic coordinates and equivalent displacement parameters for crystals of Na- MS ...... 52-58 Table 5.3 Select interactomic bond length distances for Na metaschoepite for the five refined structures, as well as those of Weller et al. (2000) ...... 60-61

Table 6.1 synthesis conditions for soddyite, uranophane, na compreignacite, becquerelite, and kasolite...... 72-73

Table 6.2 Synthesis conditions and analytical results for powders of uranophane, soddyite, and Na compreignacite synthesized at ANL...... 78-79

Table 6.3 Synthesis conditions and analytical results for kasolite and becquerelite synthesized at UND ...... 91

Table 7.1 Synthesis conditions for uranophane, kasolite, Na boltwoodite, and boltwoodite ...... 101 Table 7.2 Analysis results for uranophane, kasolite, Na boltwoodite and boltwoodite experiments ...... 110

Table 8.1 Unit cell dimensions for becquerelite ...... 126

Table 8.2 Unit cell dimensions for uranophane ...... 141

Table 8.3 Unit cell dimensions for soddyite ...... 157

Table 8.4 Unit cell dimensions for kasolite ...... 168

ACKNOWLEDGMENTS

I would like to thank my advisor Peter Burns for sharing his expertise and guidance and high level of tolerance with me, and making me a better scientist. I would also like to thank Tori Forbes and Ginger Sigmon for keeping me company on our many long drives and trips to Argonne. I would also like to thank the entire mineralogy group for their support and companionship over the years, Daniel Unruh, Valerie Gross, Nancy

Roebeck, and Jessica Beard. Thank you to John Schaeffer for assistance with the ICP and to Will Kinman for his help with the laser ablation and microprobe.

I would like to thank my family for always supporting me, emotionally as well as financially when necessary and helping me get through these longs years in graduate school. As always they bring humor and insanity to the most serious situations putting life in perspective.

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CHAPTER 1:

INTRODUCTION

1.1 Uranium and the environment

Uranium (U) is not overly abundant in nature, with an average crustal concentration of ~2.7 ppm, but is commonly found as a minor constituent in many minerals (Taylor, 1964). Furthermore, about 200 minerals contain U as an essential constituent (Mandarino, 2003). Before 1942, U was used primarily in paints and glazes.

With the knowledge gained from Marie Curie’s research, U ores were used as a source of

Ra226 for the treatment of cancer. The discovery of controlled nuclear fission of U235 led to the utilization of U as a means of nuclear energy and weaponry (Plant et al., 1999).

Uranium is also a natural source of radiation, is an anthropogenic contaminant with the potential to damage natural environments, and is of fundamental importance for radiometric dating. Continued research has been motivated by a desire to determine new and novel U compounds and phases with important material properties (Almond, 2002;

Krivovichev, 2002; Krivovichev, 2002; Locock, 2002; Locock and Burns, 2002; Burns,

2005; Krivovichev, 2005) and environmental significance (Burns, 2000; Li, 2001;

Almond, 2002; Hughes et al. 2003; Burns, 2004).

Uranium minerals are receiving renewed interest within the scientific community due to their relevance to the environment. They occur in soils contaminated with

actinides (Buck, 1996; Roh, 2000), can be bio-precipitated on U ore and mine tailings

(Macaskie et al., 2000; Brugger, 2004), and are alteration products of nuclear waste under conditions similar to those expected in the proposed geological repository at Yucca

Mountain (Finn et al., 1996; Wronkiewicz et al., 1996; Finch et al., 1999).

The formation of U deposits, which have made it possible to mine and utilize U, are products of continental crust creation. U is a large ion lithophile element (LILE), which does not fit into many of the high temperature rock forming mineral structures

(Rogers and Adams, 1969). Therefore, during the series of geochemical events that led to the creation of the continental crust, LILEs were preferentially partitioned into low- temperature, small volume melts that became progressively more concentrated in LILEs.

This resulted in igneous rocks, such as granites and rhyolites, which contain exceptionally high quantities of radioactive elements such as U (Plant and Saunders.

1996; Plant et al., 1999).

Uraninite, UO2+x, is the most common U mineral and makes up the bulk mineral content in many U deposits. Uraninite is non-stoichiometric with a defect fluorite type structure and is always partially oxidized in nature (Janeczek and Ewing, 1992).

Uraninite is an analogue for UO2 in commercial spent nuclear fuel, and natural uraninite from the Oklo, Gabon, natural fission reactions 2Ga years ago can be considered natural spent fuel (Janeczek, 1996).

In near surface and low temperature environments, U(IV) is very immobile;

2+ however, upon oxidation to U(VI), the mobile uranyl ion, (UO2) , forms. As shown in

Figure 1.1, oxidation of UO2 results in the formation of a variety of uranyl complexes with the details depending on the composition of the system. Bacteria may play a role in

the oxidation state of U and therefore its mobility (Lovely et al., 1991). Bacteria have been used to increase acid leaching of U from ores (Tuovinen and Kelly, 1974), and also for the bioremediation of water contaminated by U (Lovely and Philips, 1992). Through the catalysis of redox reactions microorganisms can have an impact on the mobility of uranyl aqueous complexes, and therefore may have played a role in U ore deposition

(Suzuki and Banfield, 1999).

Figure 1.1 Eh-pH diagram for U under synthetic J-13 water conditions at 25°C. The diagram shows the stability fields of predominant aqueous U species (Johnson and Werme, 1994).

Uranium mill tailings contain large volumes of radioactive sand-like residue resulting from the ore refining process. Mill tailings contain approximately 85% of the

radioactivity that was present in the unprocessed ore. The United States has more than

230 million tons of U mill tailings stored at mill sites (Morrison and Spangler, 1992).

1.2 Crystal chemistry of uranyl phases

Uranium exists in nature in three oxidation states, tetravalent (+4), pentavalent

(+5), and hexavalent (+6). U6+ minerals are major constituents of the oxidized portions of U ore deposits, where they are most commonly the alteration products of uraninite

(Frondel 1958; Finch and Ewing 1992). There are a few known U4+ minerals (e.g., uraninite, coffinite (Smits, 1989; Janeczek, 1991) and brannerite (Szymanski and Scott,

1982); Smith, 1984; Singh et al. 1990), and there are a few mixed U5+-U6+ minerals (e.g., wyarite, Clark, 1960). The U6+ minerals and phases are the most diverse, due to the unique crystal chemistry of U6+.

6+ 2+ U crystal chemistry almost invariably involves the (UO2) uranyl ion. The uranyl ion is approximately linear and occurs in crystal structures coordinated by four, five or six anions in an approximately planar arrangement perpendicular to the uranyl ion, giving Ur4 square, Ur5 pentagonal, and Ur6 hexagonal bipyrimids (Ur: uranyl ion, :

2- - O , OH , H2O), respectively. Both octahedral and distorted octahedral coordination geometries can also occur (Burns et al. 1997). A review of bond-length variations in U6+ coordination polyhedra for well-refined structures is given by Burns et al. (1997). For

6+ 6+ structures where the uranyl ion is present, (U -Our) and (U -eq) (eq= equatorial )

bond-lengths for Ur4 polyhedra are 1.79(3) and 2.28(5) Å, respectively. In Ur5

6+ 6+ polyhedra, the average bond length for (U -OUr) is 1.79(4) Å and (U -eq) is 2.37(9) Å.

6+ 6+ In Ur6 polyhedra, (U -OUr) is 1.78(3) Å and (U -eq) is 2.47(12) Å (Burns et al. 1997)

The uranyl ion has a formal valence of 2+, which results in typical bond-valences

6+ associated with each U -eq bond of ~ 0.5, ~ 0.4 and ~ 0.33 vu for Ur4, Ur5 and Ur6 polyhedra, (Burns et al. 1997) (Figure 1.2). Since the equatorial ligands are close to coplanar, and the OUr bond-valence requirements are nearly satisfied without additional bonding, the sharing of Urn polyhedron equatorial-edges and corners is common, and often results in structures with infinite sheets (Burns et al. 1996; Burns et al. 1997). In the case of infinite sheets the uranyl ion tends to be oriented approximately perpendicular to the sheet, which is then further coordinated through the interlayer cations, H+, and interlayer H2O groups. The sheet, which is primarily composed of uranyl polyhedra and other higher valence cation polyhedra, is defined as the primary structural unit. The

+ interlayer complex is the combination of cations, H2O groups and H bonding necessary to connect two structural units together.

There are four basic categories of structure units for uranyl minerals: clusters, chains, sheets, and infinite frameworks. These four structural types make up the uranyl hierarchy which was developed by Burns et al. (1996) and further expanded by Burns

(2005), in an effort to identify similarities in structural formation of uranyl phases.

Burns et al. (1996) classified a total of 106 U6+ structures as sheet structures; this number has almost doubled to 204 known sheet structures as of 2005 (Burns, 2005).

Sheets were compared and grouped on the basis of the topological arrangement of the anions that occur within the sheet (Burns et al. 1996; Miller 1996). All anion topologies may be regarded as arrays of edge-sharing, space-filling polygons with translational symmetry. Burns et al. (1996) identified twenty-nine unique anion-topologies. A potential cation site corresponds to each polygon in an anion topology.

O2- U6+ O2-

A.)

B.)

Figure 1.2 A) The typical uranyl UO2 ion with an average bond length of ~1.79Å to the apical O. B) The further coordination by four, five and six equatorial ligands to create square, pentagonal, and hexagonal bipyramids with average bond lengths of ~2.28Å, 2.37Å, and 2.47Å respectively (Burns et al. 1997).

As of 2005 there are 43 known compounds with structures containing finite clusters of uranyl polyhedra, 7 of these are minerals; 57 structures that contain infinite chains of polyhedra of higher bond-valence, 10 of these correspond to minerals; and 56 known infinite framework structures, 2 of these are minerals (Burns, 2005).

1.3 Natural analogues of nuclear waste

Natural U occurrences that are broadly similar geologically to Yucca Mountain have been studied as analogues for disposal of nuclear waste in Yucca Mountain (Pearcy,

1994; Pourcelot, 1998; Fujii, 2000; Jensen, 2001; Salas, 2004; Evins, 2005). The natural oxidation of uraninite produces a variety of alteration phases that are expected to be similar to those that form on altered spent nuclear fuel (Bracke, 2001).

The discovery of naturally depleted U was made in 1972 at a U enrichment plant in Pierrelatte, France (Bodu et al. 1972). Isotopic analysis of the ore revealed fission products, which verified that spontaneous nuclear chain reactions occurred at Oklo,

Gabon (Neuilly et al. 1972). The Oklo and Bangombe natural fission reactors are believed to have formed ca. 2Ga. ago in the Franceville basin, Gabon (Evins, 2005). The reaction of uraninite in these natural reactors to different geological conditions has been used as a model for the disposal of spent nuclear fuel (Johnson and Shoesmith, 1988;

Janeczek, 1995). The uraninite and zircon found in these natural reactors have been studied extensively to establish the chronology and effect of alteration events upon these deposits. The U ore formation occurred during the early stages of burial and digenesis.

Reducing fluids enriched with organic material instigated uraninite precipitation following interaction with oxidizing basin fluids (Gauthier-Lafaye and Weber, 1989).

Another significant natural analog is the Nopal I deposit within the Pena Blanca district in Chihuahua, Mexico. This is considered to be the most directly comparable natural analogue to Yucca Mountain (Pearcy, 1994). Primarily composed of uraninite, this deposit has been altered, resulting in a suite of secondary uranyl minerals. The deposit is located in a Basin and Range horst composed of welded silica tuff, a similar geologic setting to Yucca Mountain (Pearcy, 1994). The uraninite is fine grained material with a low trace element content, and interacts with ground water with a chemical composition derived from interaction with the host material, resulting in groundwater rich in Si, Ca, K, Na and OH (Pearcy, 1994). This groundwater has a composition similar to the water expected to come in contact with the stored spent fuel in the proposed repository (Pearcy, 1994).

The Nopal I deposit was created by hydrothermal solutions precipitating the U as they moved through the fractured zone within the welded silicic tuff at about 43.8 Ma

(Alba and Chavez 1974). The primary uraninite at the Nopal I deposit was continuously altered by groundwater. The first phase to appear as an alteration product was ianthinite

4+ [U2 (UO2)4O6(OH)4(H2O)4](H2O)5, which was then followed by the uranyl silicates soddyite, (UO2)2(SiO4)(H2O)2, uranophane, Ca[(UO2)(SiO3OH)]2(H2O)5, weeksite,

K2(UO2)2(Si5O13)(H2O)3, and boltwoodite, K[(UO2)(SiO3OH)](H2O)1.5. Boltwoodite and weeksite are believed to have been the last to form, or to be the most recently formed

(Leslie et al. 1993; Pearcy, 1994).

In a study by Pearcy (1994) the alteration products at Nopal I were compared to results of laboratory experiments conducted using unirradiated UO2 (Wilson, 1990;

Wilson, 1991; Wronkiewicz et al. 1992). The comparison indicated a close correlation

between the Nopal I deposit and alteration of unirradiated UO2 in experiments under moist unsaturated oxidizing conditions. Any deviations in secondary phase formation could be attributed to differences in the availability of various cations due to the constraints of using EJ-13 well water (taken from a well located close to the Yucca

Mountain repository) in laboratory experiments as compared to the oxidative groundwater in contact with the Nopal I deposit (Pearcy, 1994).

1.4 Uranium mineralogy and nuclear waste

Spent nuclear fuel (SNF) is fuel that has been removed from a nuclear reactor after being irradiated; it consists primarily of U and of radioactive nuclear fission products and transuranic elements. Commercial spent nuclear fuel (CSNF) was used in nuclear reactors to generate electricity. The DOE (Department of Energy) is responsible for determining a final solution for the management of spent nuclear fuel and other high- level radioactive waste. The majority of DOE’s spent nuclear fuel comes from defense production reactors, naval propulsion plant reactors, and a number of experimental and test reactors. In addition, DOE is responsible for the disposal of weapons-usable Pu, which is surplus to the national security needs. As of 2003, there was approximately

49,000 metric tons of spent nuclear fuel in short term storage in the United States. The waste is stored on reactor sites in over 125 locations in 39 different states throughout the

United States (DOE, 1988). The DOE’s proposed action is to construct, operate, monitor and ultimately close a geologic repository for the disposal of approximately 70,000 metric tons of spent nuclear fuel and high-level radioactive waste at Yucca Mountain, NV

(Report, 1995). Using the natural geological features of the mountain combined with

engineered barriers, DOE hopes to ensure the long-term isolation of these materials from the environment (TSPA-VA 1998).

The fuel assembly design of a general commercial reactor consists of vertically stacked UO2 pellets contained within a zirconium metal jacket known as Zircaloy

(Johnson and Shoesmith, 1988; Ewing et al. 1995; Wronkiewicz and Buck, 1999). In order to reach criticality during reactor operation, nuclear fuel for use in commercial reactors in the U.S.A. is enriched in U235 to approximately 3-5%. The fuel pellets are synthesized from granules of U oxide that have been hot-isostatically pressed. The purpose of this process is to achieve ~95% maximum theoretical density (Wronkiewicz and Buck, 1999). Commercial spent nuclear fuel generally contains between 95 to 99%

UO2, as well as fission products and transuranic elements created during nuclear power production. The elevated temperatures the fuel grains experience during nuclear reactor operation (up to 1700C) could lead to grain growth and segregation of fission products, which are not compatible with the UO2 fluorite structure. Some of these fission products will depart the UO2 matrix and follow the temperature gradient to collect within grain boundaries and the gap region between the Zircolay container and the fuel pellets. This movement of fission products is directly related to the burnup history of the fuel; increased segregation will result from an increased percent burnup (Oversby, 1994;

Wronkiewicz and Buck, 1999).

Initially DOE had planned to open a “hot” repository, in which the emplacement of the nuclear waste into the proposed repository was expected to result in a “thermal event” that will heat the repository, in the immediate surrounding host rock to the emplaced waste, to a temperature above the local boiling of water at ~96C (Bushcheck

et al. 1996). Due to this intense heating of the host rock, water is not expected to come into contact with the waste packages until the temperature falls below the boiling point.

However there is debate about allowing a “hot” repository to be opened. It has been suggested by DOE that the operating temperature of the repository not exceed 85˚C

(OCRWM 2001). In the case of a “hot” repository, following the decrease in heat the repository is expected to have virtually 100% relative humidity that will give a thin film of water on the surface of everything stored in the repository. For the case of a cooled or controlled temperature repository there is still a high probability of high humidity levels and the resulting film of water present on the emplaced waste (Whipple, 2006).

Commercial spent nuclear fuel will alter in the conditions expected at Yucca

Mountain. The oxidation of spent fuel will initiate on the grain boundaries of the fuel grain and progress inwards towards the core of the grain (Taylor and Burgess, 1980;

Thomas et al. 1989; Einziger et al. 1992). As UO2 is oxidized to U4O9 there is a slight shrinkage due to the difference in unit cell parameters that may then lead to weakened grain-boundary regions and potentially to microfracturing of the grains (Johnson and

Shoesmith, 1988). This microfracturing could develop into a pathway for water to enter the fuel grain and thereby increase the surface area of the grain exposed to fluids, which would in turn expose any fission products accumulated in gaps and along grain boundaries and may lead to an increased release rate (Ahn, 1996). Microfracturing has been shown to induce a “friable nature” to spent fuel samples in laboratory tests, which could be expected to lead to the disaggregation of fuel pellets with exposure to water

(Gray and Strachan, 1991).

The U that is released during the oxidative dissolution of spent fuel may precipitate as secondary uranyl phases when the rate of transport away from the surface is sufficiently slow, as is expected under repository conditions (Murphy and Pearcy, 1992).

The natural occurrence of uranyl minerals found as alteration layers surrounding uraninite confirms that the oxidative dissolution of uraninite can be sufficiently rapid in relation to the rate of transport (Murphy and Pearcy, 1992). The occurrence of uranyl phases as alteration products on unirradiated UO2, SIMFUEL, and spent fuel have been reported in several experimental corrosion studies (Wronkiewicz et al. 1992; Diaz-Arocas and Garcia-Serrano, 1997; Finch et al. 1999). It has been speculated that the formation of secondary uranyl alteration phases may create a protective barrier that may limit the transport of dissolved species, oxidants and other radionuclides from the surface of the fuel grains (Thomas and Till, 1984; Shoesmith, 1996). Insight into the behavior of radionuclides of concern within the repository and their interaction with uranyl alteration phases will increase the use of realistic parameters in models used to determine long-term repository behavior, thereby giving a potentially more valid view of repository behavior.

1.5 Np+5 crystal chemistry

Np-237 is a by-product of nuclear reactors and plutonium production (Stwertka,

1996). Np-237 has the longest half-life of the transuranics, which makes it of particular concern for the long-term performance of the proposed repository at Yucca Mountain

(Crowley, 1997).

As shown in Figure 1.3, Np-237 remains a steady contributor of radiation in a geological repository (Crowley, 1997). Burns et al. (1997) first predicted the incorporation of transuranic cations with a +5 or +6 charge into structures containing U+6 on the basis of chemical and structural correlations, which exist amongst certain actinides, such as Np5+,

Pu5+, and Pu6+. The potential for actinide containment within common U6+ structures that will form in a repository when spent nuclear fuel is altered would reduce the future mobility of the incorporated radionuclide.

Np5+ is expected to be the dominant oxidation state for Np within oxidizing repository conditions; it is also the most stable oxidation state of Np in solution. Np4+ exists within reducing solutions and it is possible that it may be present in some quantity

5+ within oxidized solutions. There is an argument that NpO2(cr) may form in Np rich solutions, implying Np5+ structures would be metastable (Roberts et al. 2003). While there may be regions within the spent nuclear fuel in which reducing conditions exist and therefore Np4+ would be prevalent, the general study of Np-237 aqueous species indicates that Np5+ would be the most stable aqueous form in the oxidizing Yucca Mountain repository conditions (Nietsche, 1991). The Np5+ cation is part of a nearly-linear

5+ 1+ 5+ 1+ (Np O2) ion. Like the uranyl ion, the (Np O2) ion is usually coordinated by four, five or six ligands, forming square, pentagonal and hexagonal bipyramids, respectively

(Burns et al. 1997). Forbes (2007) studied 43 inorganic Np5+ structures and calculated

5+ 5+ 5+ 1+ average bond lengths of Np polyhedra. Reported Np -O bond-lengths in the (Np O2) ion average 1.83(2) Å, 1.84(3) Å, and 1.84(2) Å for neptunyl square, pentagonal, and hexagonal bipyramids respectively, with an average (Np5+-Oeq) of 2.37(9) Å, 2.46(7) Å, and 2.54(1) Å for neptunyl square, pentagonal, and hexagonal bipyramids respectively

(Forbes, 2007) (Figure 1.4). Np5+ structures are dominated by pentagonal bipyramids, corresponding 32 of the 43 structures examined.

A.) O2- Np5+ O2-

B.)

+1 +1 Figure 1.4 (A) The (NpO2) ion with an average bond length of ~1.84 Å. (B) The (NpO2) ion further coordinated by four, five and six equatorial ligands to create square, pentagonal, and hexagonal bipyramids.

On the basis of similar average bond lengths and general coordination geometries it has been suggested that Np5+ crystal chemistry will be remarkable similar to U6+ crystal chemistry. Although the chemistry of neptunium has been researched extensively, research into neptunyl inorganic crystal chemistry is limited and there are only 43 Np5+ inorganic structures known to date as compared to the more than 360 known inorganic uranyl structures. The difference in valence state of Np5+ versus U6+ is significant, but it is difficult to predict how this charge difference will affect the structural diversity of neptunyl complexes. The fundamental reasons behind why certain structures form over others, that may be seemingly as energetically favorable or more so, is still unclear.

While there are notable similarities between the uranyl ion and the neptunyl ion that may allow for substitution of Np5+ in U6+ structures, the current state of knowledge in neptunyl crystal chemistry indicates experimental verification is essential.

In an endeavor to further understand Np5+ crystal chemistry, Forbes (2007) synthesized and structurally characterized 17 inorganic Np5+ structures. There are some similarities in structural units containing Np5+ and U6+, for example

6+ Ba(NpO2)(PO4)(H2O), which contains a sheet that is topologically identical to the U mineral uranophane (Forbes and Burns, 2006). Forbes (2007) found a marked tendency towards “cation-cation” interactions in Np5+ structures. Cation-cation interactions in the solid state were first observed by Cousson et al. (1984), and refer to the situation in which

a OAn of an (AnO2) (An refers to a an actinyl cation) ion is further coordinated by bonding to a neighboring An polyhedron, forming one of its equatorial ligands (Sullivan et al. 1961). Cation-cation interactions are observed in various Np5+ compounds, but are very unusual in the case of U6+ (Sullens et al., 2004). According to Forbes (2007), cation-

cation interactions occur in approximately 50% of the Np5+ structures, but only in 2% of

U6+ structures. The reasons for Np5+ departure from uranyl crystal chemistry via cation- cation interactions is mostly the effect of the lower charge of the neptunyl ion, as

6+ compared to the uranyl ion. In the U uranyl ion, the OUr atoms receive a valence of ~1.7 from the bond to U6+, which nearly satisfy the atoms requirement. However, in the case

5+ 5+ of the Np ion, the ONp receives only ~1.5 from the Np cation (Burns et al., 1997).

This reduction in valence is significant enough to encourage further polymerization through the ONp atom.

1.6 Thermal stability of U6+ phases

2+ The majority of minerals that contain the uranyl ion, UO2 , also contain a

- significant quantity of structurally bound H2O or OH ions (Cejka and Urbanec, 1990).

Many uranyl minerals are secondary alteration products of uraninite that form in near- surface environments and the structurally bound H2O groups can greatly influence the structural stability of these minerals.

The manner in which H2O groups are bonded within a structure influences the stability of the structure. Hawthorne et al. (2007) describes the different crystal chemical roles of H2O in common uranyl minerals in detail, as well as their potential impact on the different segments of the structure. For example, a type of water that may increase phase stability is a transformer H2O group, which bonds with a cation in the interlayer complex and to the structural unit by H bonding. If this water is lost it may have a significant impact on the stability of the structure. H2O groups that do not bond to a cation in the interlayer complex or to the structural unit may have a lesser impact on the structural

stability, as it may not be integral to the structural unit or the interlayer complex. There also exist H2O groups that bond only to the interlayer complex or the structural unit and the effect of losing this type of H2O group may vary greatly depending upon the details of the structure involved. In the case of sheet structures, the strength of bonding between the structural unit and the interlayer complex is crucial in determining the durability and stability of a sheet structure and H2O groups play a key role in understanding this relationship.

There are several processes and reactions that can be utilized to characterize secondary U minerals in relation to their thermal stability – oxidation of U4+ or Fe2+, dehydration, dehydroxylation, decomposition to an anhydrous phase, crystallization of a new phase, and reactions in the solid state between decomposition products (Cejka and

Urbanec, 1990). A variety of thermal analysis techniques have been used to examine decomposition and water loss during the heating of select uranyl minerals. According to

Cejka and Urbanec (1990) and Cejka (1999), uranyl hydroxides that do not contain cations other than U generally decompose at higher temperatures with additional complexity than other uranyl minerals, though exceptions are known to exist, such as schoepite.

Examination of dehydration of a uranyl mineral involves detailing the temperature intervals over which different types of H2O groups are lost and the distinction of true dehydration from dehydroxylation. Often the first water to be lost with increasing temperature is the water bonded in the interlayer, as it is the most loosely bonded water present (any absorbed water present would be the first released H2O Cejka (1999)).

Foldvari et al. (1988) speculated that thermal analysis could be used to determine different types of bonding of water in uranyl minerals.

With the use of thermal analysis, in addition to water loss, temperature intervals for standard changes in anhydrous phases can be established, and solid decomposition products may be characterized, as well as any reactions that occur between these decomposition products. The interlayer H2O groups may be released in a step-wise fashion or continuously, and occasionally both can occur within a mineral, for example, hydrogen meta-autunite (Sidorenko et al., 1985; Cejka, 1999).

Though a number of generalized experiments on the decomposition of uranyl minerals have been completed, there is still a wealth of information that thermal analysis could provide. Cejka (1999) discusses the existence of anhydrous phases, and structural changes upon heating, but there is no X-ray diffraction (XRD) data to identify these phases, and there is no information in the literature concerning the possible reversibility of dehydration reactions in uranyl minerals. Differential thermogravimetric analysis

(DTA) and infrared spectrometry give imprecise temperatures for dehydration events; using powder XRD to track structural changes can give us more precise information at which temperatures water is released from within a structure.

Sidorenko et al. (1985) and Cejka (1999) both conducted thermal analysis for many uranyl minerals before their structures were known, using natural mineral samples.

Without structural information, thermal analysis is an assumption of water loss as indicated by DTA and infrared spectroscopy without identification of the structural changes, and how the loss of each water molecule affected the overall crystal structure.

Thermogravimetry (TG) is a technique in which the mass of a material is measured as it is exposed to increasing temperatures. The mass is plotted versus the temperature, giving a TG curve, which gives information concerning water loss as the material is heated. This method has been used for many of the uranyl phases by

Sidorenko et al. (1985) and Cejka (1999) in an attempt to determine the temperature and pattern of water loss in a structure. For example, TG was performed on studtite, which was synthetically created for purity; the data shows a loss of 2 H2O up to 145ºC and

2H2O between 145ºC and 300ºC. Cejka et al. (1996) were unable to determine whether water and oxygen were lost simultaneously or individually during the second dehydration step. By combining TG data with powder XRD data at these critical temperature ranges I should be able to determine what is occurring structurally and better understand the entire dehydration, dehydroxylation and destruction process.

In addition to the obvious contribution this research has to understanding how uranyl structures behave when exposed to varying heating conditions, this research could have important implications for understanding the stability of uranyl alteration phases forming on the surface of spent nuclear fuel within a repository. Phases within a spent fuel rod may be exposed to a series of wetting and drying events at a range of temperatures.

1.7 Hypotheses

1. The radionuclide Np+5 can be incorporated into the structures of some uranyl

phases by substitution for U6+, and will require appropriate charge balancing

mechanisms

2. Temperature and pH will play an important role in determining the quantity of

Np+5 incorporated into a given uranyl phase. Each phase may be uniquely

impacted by temperature and pH and will need thorough examination to

determine these conditions. Temperature and pH play an important role in phase

formation and crystallinity, the exact effect of temperature and pH upon each

known structure is as yet unknown.

3. Analysis using powder XRD and thermogravimetry (TG) will permit the tracking

of structural changes in key uranyl phases as a function of temperature. With this

method I can determine phase changes, onset temperatures of dehydration,

reversibility, and kinetics for select uranyl phases. In addition, identifying the

structural changes in a phase following dehydration events, for example collapse

of the structure as compared to the destruction of the structure or a structural

transition to an anhydrous form. Powder XRD combined with the TGA and

thermal data will be used to explore the role of H2O in uranyl structures and

identify temperatures at which the stability of an individual phase can be lost.

CHAPTER 2:

MATERIALS AND METHODS

This chapter describes the techniques used for material characterization, crystal structure determination, and other methods used during the experimental execution of this research.

2.1 X-ray Powder diffraction

Powder diffraction experiments reported in this dissertation were conducted using an automated Scintag X-ray diffractometer. Powders synthesized without Np5+ were prepared for analysis by taking an aliquot of sample (~3-10 mg), which was then ground to a fine powder and deposited on a zero-background quartz plate. Samples that contained Np that were analyzed at the University of Notre Dame (UND) were prepared similarly. After the powder was deposited on the quartz plate the entire plate was wrapped securely in plastic wrap. The plastic wrap contributes only one peak at 20˚ two- theta that was subsequently subtracted from each powder X-ray diffraction (PXRD) pattern. PXRD patterns were collected for each powder at UND using a Scintag theta- theta diffractometer and CuK radiation. Patterns were collected over the two-theta range 10-90° with a scan rate of 0.5° per minute. For powders synthesized and prepared at Argonne National Laboratory (ANL), sample preparation included a modified stage attachment that securely contains all Np-bearing powder. Powders analyzed at ANL were

also analyzed using a Scintag theta-theta diffractometer and CuK radiation over the two-theta range 2-90° with a scan rate of 0.25° per minute. The resulting powder diffraction patterns were compared to those provided in the Powder Diffraction File, as well as those calculated using the corresponding crystal structures such as for Na compreignacite, which is absent from the Powder Diffraction File.

2.2 Single crystal X-ray diffraction

Single crystal diffraction data was collected for suitable single crystals on a three- circle Bruker single crystal X-ray diffractometer equipped with monochromatic MoKα (λ

= 0.71073 Å) radiation and a 4K APEX detector, with a crystal-to-detector distance of

4.67 cm. Unit cell parameters were determined using the Bruker software SMART

(Bruker, 1998) and the data was integrated using SAINT (Bruker, 1998). Lorentz and polarization corrections were applied to the data using the Bruker program XPREP

(Bruker, 1998). Absorption corrections were performed using a semi-empirical model and the programs XPREP or SADABS. The structures were solved using direct methods and refined on the basis of F2 for all unique data using the Bruker SHELXTL Version 5 system of programs (Bruker, 1998). Atomic scattering factors for each atom were taken from International Tables for X-ray Crystallography (Ibers and Hamilton, 1974).

2.3 Inductively coupled plasma-mass spectrometry: ICP-MS

An Element 2 was used for the liquid ICP-MS analysis for Np-237, U-235 and U-

238 for samples analyzed at UND. The Element 2 consists of an advanced double- focusing magnetic sector ICP-MS. Medium and high resolution modes provide separation

of the analyte signal from spectral interferences, making accurate and precise elemental analysis of trace elements in complex matrices possible. Due to unexpected complications in analyzing Np in liquid form, a teflon spray chamber and teflon tubing was used for sample introduction. An extensive wash protocol was used to ensure removal of Np from the system. Seven minutes of total wash time between samples was used, the first wash consisted of a revolving wash of 10% nitric and 10% hydrochloric acid with 10 drops of HF.

Samples for ICP-MS analysis were dissolved in 2M HCl and diluted to an appropriate range for analysis that will ensure detection without over-saturating the detector. For this research the ICP-MS is used primarily to detect Np-237 within powder samples. Once the powders were dissolved they were diluted to a range between 0.1-1 ppb Np with a corresponding calibration range of 0.1-5 ppb using a National Institute of

Standards and Technology (NIST) approved Np standard 4341.

ICP-MS data collection conducted at ANL for Np-237 was determined by inductively coupled plasma-mass spectrometry (ICP-MS) with a VG Elemental, Plasma

Quad II system, using thorium as internal standard. The quadruple mass analyzer was tuned to optimize resolution at mass 238 and minimize tailing of the U-238 peak into the

237 mass position. Samples were diluted to a concentration of 10 mg/L U or to a Np concentration lower than the highest calibration standard (65 microgram/L). Analysis of standards containing 10 mg/L U and various amounts of added Np in previous work had demonstrated that the low mass tail of the m/e 238 peak at this U concentration made a negligible contribution to the Np-237 signal.

Calibration standards for the Np measurements were prepared by volumetric dilution of a stock solution that was standardized by alpha counting. For this standardization, a measured aliquot of the stock solution was deposited on a planchet and the total alpha emission rate was measured with a low-background gas proportional counting system (Tennelec LB 4100) in 2-π geometry. Then the energy distribution of alpha particles from the planchet was determined with a Canberra AlphaAnalyst Alpha

Spectrometer that uses a solid-state surface-barrier detector to record the number of detected particles as a function of energy. This analysis showed that 89.2% of the emitted alpha particles arose from Np-237, 10.4% from Pu-238 or Am-241, and 0.4% from Pu-

239/240 in the Np source material. The Np concentration in the stock solution was calculated from the 2-π alpha count rate after accounting for the efficiency of the 2-π counter and the fraction of the alpha activity attributable to Np. The specific activity for

Np-237 was taken as 1565.1 dpm per microgram Np. Analyses at ANL were conducted by Yifen Tsai with coordination by Donald G. Graczyk.

2.4 Inductively coupled plasma atomic emission spectrometry: ICP-AES

Samples for ICP-AES analysis were analyzed for bulk constituent elements such as U, Ca, and Si. Samples analyzed at UND were diluted to give elemental U levels ~10 ppm using a Perkin Elmer Optima-Series Model 3300 DV. Calibration standards were prepared by gravimetric dilution of certified standards from Spex CertiPrep, Metuchen

NJ. Samples analyzed at UND recorded associated error and an calculated error for each sample was determined.

Samples analyzed at ANL for Ca, Na, and U, were performed using a Perkin

Elmer Optima-Series Model 3300 DV instrument operated in the radial viewing mode.

Calibration standards were prepared by volumetric dilution of certified solution standards procured from High Purity Standards, Charleston SC (U), Alfa Aesar, Ward Hill NJ (Na), or Spex CertiPrep, Metuchen NJ (Ca). The same dilutions prepared for the ICP-MS measurements were used in the ICP-AES analysis to keep Np concentrations in a range

(<100 dpm/mL) consistent with radiological control limits for the ICP-AES instrument.

Analyses at ANL were conducted by Seema Naik with coordination by Donald G.

Graczyk. Results are received with a ± 10% error.

2.5 Thermogravimetric analysis

The TGA used in these experiments is a Netzsch TG 209 F1 Iris. Samples were placed in a small crucible and the sample starting weight was recorded. As the sample was heated the change in mass was recorded, giving a TGA curve characteristic of each individual sample in which the total mass lost was measured and the temperatures at which this occurs are determined. For this study both temperature ramping at a constant rate of 10˚C/min, and a combination of ramp and holding segments were used. The TGA technique is used in combination with high-temperature PXRD. The sample was heated at a constant rate of 10˚C/min on the PXRD and the temperature was held constant to collect the PXRD pattern. To determine if water loss will continue when a sample is held at a particular temperature, a ramp and hold method was used and compared to the continuous heat rate curve.

2.6 High temperature stage with powder X-ray diffraction

An HDK 1.4.high temperature stage produced by Edmund Buhler was attached to the Scintag powder X-ray diffractometer. The HDK 1.4 shown in Figure 2.6 has a maximum temperature in air of 1600˚C. It contains a beryillium X-ray window 210˚ and

12 mm wide, with a sample adjustment device to raise or lower the sample to optimize beam position on the filament. The heating filament is Pt with a thermocouple attached directly to the back of the filament to give an accurate measurement of the temperature of the filament. All samples were placed directly onto the Pt filament.

2.7 pH measurements

Accurate and precise pH measurements are critically important when pH is one of the vital factors being measured pertinent to Np incorporation in synthetic solids. Several electrodes were used throughout the course of this work. Certified calibration standards were used, at pH values of 4.0, 7.0 and 10.0 from Fisher scientific. Certified Fisher

Scientific Ag/Cl electrode solution was used to fill the electrode.

2.8 Laser-ablation inductively coupled plasma mass spectroscopy

LA-ICP-MS involves the same fundamentals as solution ICP-MS, however the introduction of sample into the plasma is very different. Rather than a solution or aerosol, dry samples, such as single crystals, are directly ablated using a laser, which can be focused to ~ 4µm in diameter. For analysis of a single crystal, the crystal is placed on double sided sticky tape and ablated with the laser beam with a beam diameter as small as

~4µm, depending upon the size of the crystal. A New Wave UP 213 nm laser was used to

ablate single crystals directly into the high resolution magnetic sector ELEMENT 2 inductively coupled plasma mass spectrometer for the analysis of Np. There are currently no solid standards available for Np. To determine a rough estimate of the quantity of Np present in a crystal comparing Np counts to U-233 present in a uranyl crystal is a qualitative way and roughly close to the expected quantity of Np expected.

2.9 Electron Microscopy and Microprobe

Selected crystals were mounted, polished, and coated with C and analyzed using a

JEOL 8600 electron microprobe. Samples were also examined using an LEO EVO-

50XVP variable pressure/high humidity scanning electron microscope to determine chemical composition and to obtain images of crystals and powder grains. For challenging crystals in which SEM and electron microprobe data was difficult to quantify, an analysis was obtained from the University of Chicago, performed by Dr. Ian

Steele.

2.10 Ultraviolet Visible Spectroscopy

A Jasco V-670 spectrometer (UV-vis) was used to characterize the oxidation state of Np stock solutions at the University of Notre Dame. The oxidation state is indicated by characteristic absorptions; the area under the associated peaks provides approximate concentration. Analysis includes filling a plastic cuvette with the Np stock solution after running a background or blank cuvette of 1 M HCL to subtract out. This method is efficient and inexpensive, EXAFS would be an alternative way to determine the oxidation state of the Np stock solution, but this would require transporting Np rich

solution to ANL after obtaining beam time, and there is the possibility of reducing the Np in the beam.

CHAPTER 3:

CRYSTAL STRUCTURES

For the neptunium incorporation and thermal analysis research, structures of prevalent uranyl phases found as alteration products of uraninite, acid mine drainage waste, and spent nuclear fuel were chosen for this research. The structures described within this chapter have been thoroughly characterized and studied. The structural similarities between these uranyl phases may have an impact on how a phase incorporates Np5+and will have an impact on the thermal behavior of each phase. This chapter describes the structures of the uranyl minerals that will be discussed throughout this dissertation.

Relationships between the structures that may impact Np incorporation or thermal analysis will also be detailed.

3.1 Uranophane Sheet Topology

The uranophane group of minerals contain uranyl silicate sheets that are based upon the same topology. The uranophane sheet topology contains uranyl ions coordinated by five ligands that are arranged at the equatorial vertices of pentagonal bipyramids. The sheet contains infinite chains of edge-sharing uranyl pentagonal bipyramids that are one polyhedron wide. Silicate tetrahedra are attached to both sides of these chains by sharing edges that correspond to two equatorial vertices of the bipyramids (Figure 3.1). The silicate tetrahedra on opposite sides of the chain of bipyramids alternate, such that any bipyramid shares an edge with only one tetrahedron. Uranophane, kasolite, boltwoodite

and Na substituted boltwoodite, all are members of the uranophane group have been examined in the current study.

Figure 3.1 The uranophane-type sheet showing the linking chains of yellow uranyl pentagonal bipyrimids and turquoise silicate tetrahedra.

3.1.1 Uranophane

In the structure of uranophane, Ca[(UO2)(SiO3OH)]2·5H2O, each silicate tetrahedron shares a vertex with a uranyl bipyramid of an adjacent chain, resulting in two-dimensional uranyl silicate sheets (Figure 3.2). The fourth vertex of each silicate tetrahedron is occupied by an (OH)- group, and this ligand extends into the interlayer region that also contains the Ca cations as well as H2O groups that are either bonded to

Ca or held in the interlayer by H bonding only. The silicate tetrahedra are oriented in a way that non-apical anions of the adjacent tetrahedra along a chain alternate up and down

(Ginderow 1988).

A.)

B.)

Figure 3.2 The structure of uranophane, Ca[(UO2)(SiO3OH)]2·5H2O A.) The linking chains of yellow uranyl pentagonal bipyrimids and turquoise silicate

tetrahedra that form the uranophane-type sheet. B.) The interlayer Ca and H2O groups sandwiched between the sheets. Ca atoms are shown as the black and white checked speres, O atoms of the water groups are shown as white spheres, the H of the water groups are shown as the black spheres.

3.1.2 Boltwoodite and Na-Boltwoodite

Boltwoodite, K[(UO2)(SiO3OH)](H2O)1.5, crystallizes in space group P21/m, a

+ 7.0772 Å, b 7.0597 Å, c 6.6479 Å. K cations are located in the interlayer with H2O groups. The sheets are connected by bonding to the interlayer cations and through a complex network of H bonding (Stohl and Smith (1981) and Burns (1998)). Na substituted boltwoodite, Na[(UO2)(SiO3OH)](H2O)1.5, is considered analogous to end

member boltwoodite, but powder X-ray diffraction of the Na end member boltwoodite indicated a slightly larger unit cell (Vochten 1997).

3.1.3 Kasolite

Kasolite, Pb[(UO2)(SiO4)](H2O), crystallizes in space group P21/c, a 6.704, b

6.932, c 13.252 Å. Kasolite is unique within the uranophane group in that the apical anions of the silicate tetrahedra bond directly to the Pb2+ cations rather than forming acid silicate groups. All Pb2+ cations within the structure are coordinated by two uranyl apical

O atoms, two O atoms of silicate tetrahedra, two equatorial O ligands of the uranyl polyhedra and silicate tetrahedra, and one H2O group shown as a grey sphere shown in

Figure 3.5 (Rosenzweig and Ryan 1977).

3.2 Soddyite

2+ Soddyite, (UO2)2(SiO4)(H2O)2, contains the nearly linear (UO2) uranyl ion. The uranyl ion is coordinated by five ligands arranged at the equatorial vertices of pentagonal bipyramids. The structure contains infinite chains of edge-sharing uranyl pentagonal bipyramids that are one polyhedron wide. Silicate tetrahedra are attached to both sides of these chains by sharing edges that correspond to two equatorial vertices of the bipyramids

(Figure 3.5). The silicate tetrahedra on opposite sides of the chain of bipyramids alternate, in a way in which any bipyramid shares an edge with only one tetrahedron. For soddyite, silicate tetrahedra of one chain share an edge with a uranyl pentagonal bipyramid of an adjacent chain, resulting in a three-dimensional framework structure.

A) B)

C.)

Figure 3.3 Boltwoodite structure. A.) The side view of the sheet structure through the interlayer with all O atoms shown as red spheres, water molecules are shown as gray spheres bonding to the interlayer K atoms shown as the purple spheres. B.) C.) A planar view of the uranophane type sheet in boltwoodite with uranyl yellow pentagonal bipyramids and turquoise silicate tetrahedra.

A.) B.) C.)

A.) B.)

C.)

The H2O groups are located at unshared equatorial vertices of the uranyl pentagonal bipyramids, and H bonds provide additional linkages between the uranyl silicate chains (Demartin et al. 1992).

3.3 Becquerelite

Becquerelite, Ca[(UO2)3O2(OH)3]2(H2O)8, crystallizes in space group Pn21a, a

13.8378Å, b 12.3781Å, c 14.9238Å, V 2,556.23 Å. The structure contains two

2+ symmetrically distinct U positions. Each U cation forms the typical (UO2) ion and is then coordinated by five additional anions to form uranyl pentagonal bipyramids. The structure of becquerelite is a member of the α–U3O8-type sheet group, consisting of edge and vertex sharing uranyl pentagonal bipyramids, with monovalent cations and H2O groups in the interlayer. The sheet-type structure is composed of uranyl pentagonal

+2 bipyramids, with Ca and H2O groups in the interlayer as shown in Figure 3.6 (Pagoaga et al. 1987).

A.) B.)

Figure 3.6 A. ) A planar view of the becquerelite Ca[(UO2)3O2(OH)3]2(H2O)8, sheet B.) A side view showing the interlayer water as grey spheres and Ca2+ as blue spheres to be seen.

3.4 Na compreignacite

Na-compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7, has not been structurally characterized, but X-ray powder diffraction data indicates that it is isostructural with compreignacite (Figure 3.7). Compreignacite crystallizes in space group Pnmn, a 7.16 Å, b 12.14 Å, c 14.88 Å, V 1,293.41 Å, Z = 2. This structure contains three symmetrically

2+ distinct U positions; each forms the typical (UO2) ion that are further coordinated by five additional anions to form uranyl pentagonal bipyramids. The structure of compreignacite is a member of the α–U3O8-type sheet group, consisting of edge and vertex-sharing uranyl pentagonal bipyramids, with monovalent cations and H2O groups in the interlayer. The sheets are connected by both bonds to the interlayer cations and through a complex network of H bonding (Burns 1998).

A.) B.)

Figure 3.7 A.) Na-compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7, a planar view of the sheet of uranyl pentagonal bipyramids. B.) The interlayer connections +1 through the Na atoms shown as the black and white checkered spheres and H2O groups shows as the white spheres.

3.5 Na-substituted meta-schoepite

Na-meta-schoepite, Na[(UO2)4O2(OH)5]·5H2O, is structurally similar to meta-schoepite

(Figure 3.8). I solved its structure in space group Pbcn and established it contains four

6+ 2+ symmetrically distinct U sites. All U cations are in typical (UO2) ions that are further coordinated by five anions at the equatorial vertices of pentagonal bipyramids. The sheet

topology is an array of edge and corner-sharing uranyl pentagonal bipyramids, identical to that in metaschoepite. Unlike metaschoepite though, the Na form has a –1 charge on the sheet rather than and Na+1 cations occur in the interlayer of the structure

(Klingensmith et al. 2007).

A.)

B.)

Figure 3.8. Na-meta-schoepite, Na[(UO2)4O2(OH)5]·5H2O. A.) The uranyl pentagonal bipyrimids forming the sheet layer. B.) The bottom figure shows the +1 interlayer Na and H2O groups in red.

CHAPTER 4:

SYNTHESIS OF URANYL PHASES

Mild hydrothermal techniques were used for the synthesis of powders of soddyite, uranophane, becquerelite, Na-substituted compreignacite, boltwoodite, Na-boltwoodite, kasolite, and Na meta-schoepite used in this study. Preliminary experiments in the absence of Np were used for optimization of reaction conditions to increase yield and purity of the products. Reaction conditions involving a range of temperature and the initial pH of the mother solution were explored. Only conditions that yielded the target phases with no detectable impurities in the powder were selected for experiments including Np. Two stock solutions of Np5+ in 1 M HCl were prepared later with concentrations of 98 and 79 mM for synthesis conducted at ANL, a stock solution of

Np5+ in 1M HCl with a concentration of ~fifty mM was prepared for synthesis conducted at the University of Notre Dame.

All syntheses were prepared by placing the reactants in 7 mL Teflon cups with treaded screw-on tops. After closure the cups were placed in 125 mL Teflon-lined Parr reaction vessels to provide for secondary containment. Fifty mL of ultrapure water was added to each 125 mL vessel, in addition to two sealed 7 mL cups, in order to provide counter- pressure during heating. For thermal experiments or synthesis at UND without Np5+

present, reactants were prepared directly in 23 mL Teflon-lined Parr reaction vessels. All temperatures given are expected to accurate within ±5˚C.

4.1 Synthesis of Uranophane

Powders of uranophane, Ca[(UO2)(SiO3OH)]2·5H2O, were synthesized by combining 0.42 g of UO2(CH3COO)2·2H2O with 0.28 g Na2SiO3.9H2O and 0.28 g of

Ca(CH3COO)2·H2O in 5 mL of ultrapure H2O. Reactants were heated to 80, 100, 120,

140 and 160°C for 24 hours in a gravity convection oven, after which the vessels were removed from the oven and allowed to cool in air. For the Np5+ incorporation experiments, Np5+ stock solution was added and the pH of the resulting solutions were adjusted to one of 4.0, 5.0 or 6.0, measured using a pH electrode, using solutions of HCl.

4.2 Synthesis of Soddyite

Powders of soddyite, (UO2)2(SiO4)(H2O)2, were synthesized by combining 0.42 g of UO2(CH3COO)2.2H2O with 0.14 g Na2SiO3.9H2O in 5 mL of ultrapure H2O.

Reactants were heated to 80, 100, 120, 140, 150 and 160°C for 7 days in a gravity convection oven, after which the vessels were removed from the oven and allowed to cool in air. For Np5+ experiments, Np5+ stock solution was added to each experiment. In each case, the pH of the resulting solution was adjusted to 4.0, as measured with a pH electrode, using solutions of HCl and NaOH.

4.3 Synthesis of becquerelite

Powders of becquerelite, Ca[(UO2)3O2(OH)3]2(H2O)8, were synthesized by combining 0.42 g of UO2(CH3COO)2(H2O)2 and 0.06 g CaCO3 powder with 5 mL of ultra pure H2O. Reactants were heated to 80, 100, 120, 140 and 150°C for 24 hours. For

Np5+ incorporation experiments, Np5+ stock solution was added to each experiment. The pH values of the resulting solutions were adjusted to 5.0 as measured with a pH electrode, using solutions of HCl and NaOH. To synthesize crystals of becquerelite the same conditions are used but the CaCO3 used was in crystal form.

4.4 Synthesis of Na compreignacite

Na-compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7, was synthesized by combining

0.42 g UO2(CH3COO)2(H2O)2 with 0.04 g Na2CO3 in 5 mL ultrapure H2O. Reactants were heated to 100, 120, 140 and 160°C for 24 hours. For Np5+ incorporation experiments Np5+ stock solution was added to each experiment. The pH values of the resulting solutions were adjusted to 6.0, 6.5, 7.0 and 7.5 as measured with a pH electrode, using solutions of HCl and NaOH.

4.5 Synthesis of Kasolite

Kasolite was synthesized by combining 0.42 g UO2(CH3COO)2(H2O)2 with 0.27 g

Pb(CH3COO)2(H2O)2, and 0.25 g Na2SiO3.9H2O in 5 mL ultrapure H2O. Reactants were heated to 100, 120, 140 and 150˚C for 24 hours. For Np5+ incorporation experiments

Np5+ stock solution was added to each experiment. The pH of the resulting solutions

were adjusted to 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0, as measured with a pH electrode, using solutions of HCl and NaOH.

4.6 Synthesis of Boltwoodite

Boltwoodite, K[(UO2)(SiO3OH)](H2O)1.5, was synthesized by combining 0.42 g

UO2(CH3COO)2(H2O)2 with 0.041g Na2CO3 in 5 mL ultrapure H2O. Reactants were heated to 100, 120, 140 and 160˚C for 24 hours. For Np5+ incorporation experiments

Np5+ stock solution was added to each experiment. The pH of the resulting solution was adjusted to 6.0, 6.5, 7.0 and 7.5 as measured with a pH electrode, using solutions of HCl and NaOH.

4.7 Synthesis of Na-substituted Boltwoodite

Na substituted boltwoodite, Na[(UO2)(SiO3OH)](H2O)1.5,was synthesized by combining 0.42 g UO2(CH3COO)2(H2O)2 with 0.041g Na2CO3 in 5 mL ultrapure H2O.

Reactants were heated to 100, 120, 140 and 160˚C. For Np5+ incorporation experiments

Np5+ stock solution was added to each experiment. The pH of the resulting solution was adjusted to 6.0, 6.5, 7.0 and 7.5 as measured with a pH electrode, using solutions of HCl and NaOH.

4.8 Synthesis of Na-substituted Meta-schoepite

Na-substituted meta-schoepite, Na[(UO2)4O2(OH)5]·5H2O, was synthesized by combining 0.42 g UO2(CH3COO)2(H2O)2 and 0.30 g of natural albite crystals with 5 mL of ultrapure H2O and heated for 24 hours at 150˚C. The pH value was as mixed. There

are several different methods to synthesize Na substituted meta-schoepite and will be further discussed in Chapter 5.

CHAPTER 5:

Np5+ INCORPORATION IN Na SUBSTITUTED METASCHOEPITE

Consistent with all known uranyl oxide hydrate minerals, metaschoepite,

[(UO2)4O(OH)6](H2O)5, contains sheets of uranyl polyhedra with H2O groups located in the interlayer. Several crystals along the series from metaschoepite to Na-substituted metaschoepite (Na-MS), approximate formula Na[(UO2)4O2(OH)5](H2O)5, have been synthesized and their structures have been determined. Synthesis of crystals of Na-MS

5+ from a solution containing ~500 ppm Np , followed by analysis of the crystals using laser-ablation inductively-coupled-plasma mass-spectroscopy, was used to examine Np5+ incorporation in these crystals.

5.1 Crystal Synthesis

5.1.1 Synthesis of Na-substituted metaschoepite

Crystals of Na-MS obtained in four different synthesis experiments were selected for study by X-ray diffraction. Crystals (designated Na-MS-CRY) were synthesized by reacting 4 mL of a 0.2 M UO2(CH3COO)2(H2O)2 solution with 0.30 g of crystal fragments of natural cryolite. The solution pH was adjusted to 4 by addition of dilute

HCl. The solution and cryolite were placed in a 23 mL Teflon-lined reaction vessel that was sealed and heated at 120ºC for 7 days. Slow dissolution of the cryolite crystals

provided for nucleation and growth of tabular crystals of Na-MS-CRY that reached 200

μm in maximum dimension. To examine the possible incorporation of trace levels of

Np5+ into the structure of Na-MS, the synthesis using cryolite was repeated with the

solution spiked with ~500 ppm Np5+ (designated Na-MS-CRY-Np).

During the course of studying the interaction of uranyl bearing solutions with crystals of natural albite, crystals of Na-MS formed. The crystals of Na-MS appeared on crystals of albite that had been soaked in a 0.001 M solution of UO2(CH3COO)2(H2O)2

(designated Na-MS-AB1) and in a 0.01 M solution of UO2(CH3COO)2(H2O)2

(designated Na-MS-AB2) sealed in 23 mL Teflon-lined reaction vessels at 80ºC for two weeks. Formation of the Na-MS crystals was promoted by the slow dissolution of albite, which released Na into solution.

5.1.2 Natural Specimen

A crystal of natural metaschoepite was acquired from the Harvard museum. The specimen was uncatalogued and is labeled “Paraschoepite – Schoep type material,

Katanga, Congo”. No further information is known concerning the history of the specimen. Paraschoepite was defined by Schoep and Stradiot (1947) with the formula

5UO ·9½H O. Christ and Clark (1960) reported paraschoepite from a sample that also 3 2 contained schoepite. Metaschoepite was also described by Christ and Clark (1960) with the formula UO3.2H2O. Considerable progress has been made in understanding the structures and chemistry of both schoepite (Finch et al. 1996) and metaschoepite (Weller et al. 2000), and both have been synthesized. The unit-cell dimensions obtained for the natural crystal studied here (Table 5.1) are consistent with metaschoepite but not with

those of paraschoepite provided by Christ and Clark (1960). Further details concerning the distinction of the various schoepite-like minerals are provided by Finch et al. (1998).

5.2 Chemical Analysis

Samples were first examined using an LEO EVO-50XVP variable pressure/high humidity scanning electron microscope. An SEM image of crystals of Na-MS-AB2 is provided in Figure 5.1. Energy dispersive spectroscopy verified the presence of U and Na in the crystals, and also revealed the presence of F in crystals synthesized in the presence of cryolite.

Selected crystals were subsequently mounted, polished, and coated with C for analyses using a JEOL 8600 electron microprobe by Dr. Ian Steele at the University of

Chicago. Standards used were synthetic UO , synthetic CaF , and Amelia albite. Two 2 2 grains were analyzed for each material. The UO concentrations obtained from the 3 microprobe were consistently higher than expected (ranging from 90.4 to 95.6 wt.%), indicating loss of volatiles that presumably include H O and Na that are located in the 2 interlayer of the structure. It is possible that the volatiles were quickly lost in the vacuum of the instrument, during sample preparation, or during irradiation with the electron beam. The analyses for Na are not considered reliable, but they did confirm the presence of Na in each crystal. In the case of Na-MS-CRY the analyses also indicated from 1.4 to

2.0 wt.% F in the crystals (Na-MS-CRY-Np was not analyzed using the electron microprobe).

TABLE 5.1

CRYSTALLOGRAPHIC AND REFINEMENT INFORMATION FOR CRYSTALS OF Na-MS

Natural Na-MS-CRY Na-MS-CRY- Na-MS-AB1 Na-MS-AB2 Weller et al.

Np (2000)

a (Å) 14.6801(16) 14.7050(6) 14.6401(15) 14.6317(27) 14.6592(9) 14.6861(4) b (Å) 14.0287(15) 14.0565(5) 14.0417(14) 14.0147(25) 14.0358(8) 13.9799(3)

c (Å) 16.7196(17) 16.7051(6) 16.7044(17) 16.6977(30) 16.7148(10) 16.7063(5) 3 V (Å ) 3443.28 3452.96 3433.96 3424.01 3439.13 3429.97

48 Space Group Pbcn Pbcn Pbcn Pbcn Pbcn Pbcn

2max() 69.2 69.1 69.1 69.4 69.0

Total reflections 59442 64326 61804 63926 32006

Unique reflections 7190 7295 7247 7249 7059

Data with F  4(F) 3399 4349 2482 2908 3718

R1 (%) 5.49 3.45 5.28 6.24 3.92

wR2 (%) 10.34 6.15 7.83 12.15 6.97 S 1.13 0.93 0.73 0.84 0.77

Parameters 124 194 164 194 194

2 2 2 2 2 ½ 2 1/2 R1 = (Fo- Fc)/Fo x 100 ; wR2 [w(Fo -Fc ) /wFo ) ] ; S = [w(Fo- Fc) /(m-n)] 2 2 2 2 2 w = 1/[ (FO ) + (0.0314 x P) ], P = (max(FO ,0) + 2 x FC )/3

Figure 5.1 SEM image of Na-MS crystal.

Determination of the Np concentration in Na-MS-CRY-Np presents a significant challenge, as spatial resolution and high sensitivity is needed for the examination of the single crystals. The analysis is further complicated by the need to quantify a few parts per million of Np in a sample containing tens of weight percent U. A New Wave UP 213 nm laser was used to ablate single crystals directly into a high resolution magnetic sector

ELEMENT 2 inductively coupled plasma mass spectrometer for the analysis of Np. Prior to analyzing the Np-bearing crystal, crystals grown in the absence of Np (Na-MS-CRY) were analyzed to verify that there is no interference of the 238 mass isotope of U with the

237 mass window selected for the analysis. The results for a crystal of Na-MS-CRY-Np are shown in Figure 5.2.

5.3 Structure solution

5.3.1 X-ray data collection

Single crystal X-ray diffraction data were collected for each of the four synthetic materials and for a natural crystal of metaschoepite using a Bruker three-circle diffractometer equipped with an APEX CCD detector, graphite-monochromatized MoKα radiation, and a crystal-to-detector distance of 4.67 cm.

Figure 5.2 LA-ICP-MS data for a crystal of Na metashoepite. The image on the top left shows the ablation tracks of a 15µm laser.

A sphere of three-dimensional data was collected for each crystal except Na-MS-

AB2, for which more than a hemisphere of data was collected. The diffraction data were collected using frame widths of 0.3º in ω and from 20 to 30 seconds spent counting per frame. Unit-cell dimensions were refined by least-squares techniques using the positions of reflections selected from the data. Intensity data were corrected for Lorentz, polarization and background effects using the Bruker program SAINT. Data were corrected for absorption semi-empirically by modeling the crystals as plates; reflections having a plate-glancing angle less that 3° were discarded. Crystallographic details for each crystal are provided in Table 5.1.

5.3.2 Structure refinement

On the basis of the similarity of the unit cells of Na-MS-CRY and synthetic metaschoepite, refinement of the structure was commenced using space group Pbcn and the atomic coordinates for synthetic metaschoepite from Weller et al. (2000) as the starting model. Inspection of the preliminary model revealed that the O20 and O21 sites of metaschoepite, which are occupied by H2O located in the interlayer, were absent in

Na-MS-CRY. However, the locations of the U atoms and O atoms from O1 to O19 of metaschoepite had equivalent sites in Na-MS-CRY. Inspection of difference-Fourier maps calculated for the partial-structure model revealed four additional sites in the interlayer region. Each of these sites exhibits some degree of positional disorder and partial occupancy, as evidenced by relatively large refined displacement parameters. The

final structure model, which was refined on the basis of F2 for all unique reflections, included all atomic positional parameters, anisotropic displacement parameters for the U sites and O sites number from O1 to O14, an isotropic displacement parameter for the

O15 site, and fixed isotropic displacement parameters and refined occupancy factors for

O sites numbered O16 through O22. The O16 atom is distributed over two closely spaced sites designated O16a and O16b. The O21 site is displaced off the special position, such that symmetrically equivalent O21 sites are separated by ~0.8 Å, and only one can be occupied locally. Crystallographic parameters and refinement statistics are presented in

Table 5.1 and atomic positional parameters and site occupancy factors are in Table 5.2.

TABLE 5.2 ATOMIC COORDINATES AND EQUIVALENT DISPLACEMENT PARAMETERS FOR CRYSTALS OF Na-MS ______Natural MS Na-MS-CRY Na-MS-CRY-Np Na-MS-AB1 Na-MS-AB2 Weller ______

U(1) x 0.23381(4) 0.23566(2) 0.23574(4) 0.23479(4) 0.23414(3) 0.23453(4) y 0.74397(3) 0.73997(2) 0.74017(4) 0.74265(4) 0.74380(2) 0.74724(4) z 0.37478(3) 0.38224(1) 0.38168(3) 0.37739(3) 0.37476(2) 0.36301(4) U(eq) 0.01545(12) 0.01284(6) 0.01471(1) 0.01824(1) 0.01425(8) 0.01561(18) Occupancy 1 1 1 1 1 1 U(2) x 0.28634(4) 0.27743(2) 0.27735(4) 0.28370(5) 0.28636(3) 0.29656(5)

y 0.76845(3) 0.76403(2) 0.76422(4) 0.76835(4) 0.76851(2) 0.77012(4) z 0.60325(3) 0.61003(2) 0.60929(3) 0.60448(3) 0.60317(2) 0.59321(4) U(eq) 0.01605(12) 0.01368(6) 0.01475(1) 0.01861(1) 0.01568(8) 0.01655(19) Occupancy 1 1 1 1 1 1 U(3) x 0.25577(5) 0.25494(2) 0.25531(5) 0.25685(5) 0.25637(3) 0.25426(5) y 0.51214(3) 0.51522(2) 0.51527(4) 0.51221(4) 0.51220(3) 0.51155(4) z 0.23933(3) 0.24274(1) 0.24250(3) 0.24135(3) 0.23921(2) 0.22973(4) U(eq) 0.01607(13) 0.01410(7) 0.01534(1) 0.01698(1) 0.01576(9) 0.01706(19) Occupancy 1 1 1 1 1 1 U(4) x 0.25591(5) 0.25380(3) 0.25387(6) 0.25692(6) 0.25652(3) 0.25432(5) y 0.51731(3) 0.51705(2) 0.51696(4) 0.51835(4) 0.51717(2) 0.51460(4) z 0.51083(3) 0.51584(1) 0.51512(3) 0.51248(4) 0.51109(2) 0.50131(4)

TABLE 5.2 (continued) U(eq) 0.01707(12) 0. 01329(6) 0.01435(1) 0. 02173(1) 0.01603(9) 0.01630(19) Occupancy 1 1 1 1 1 1 O(1) x 0.1748(8) 0.1613(4) 0.1684(7) 0.1700(9) 0.1683(5) 0.1802(9) y 0.7406(6) 0.7403(3) 0.7423(7) 0.7462(7) 0.7458(4) 0.7479(7) z 0.6145(6) 0.6273(3) 0.6263(6) 0.6131(8) 0.6126(4) 0.5935(8) U(eq) 0.0347(2) 0.0321(1) 0.0295(2) 0.0413(3) 0.0316(1) 0.032(3) Occupancy 1 1 1 1 1 1 O(2) x 0.1475(8) 0.1407(4) 0.1449(8) 0.1506(1) 0.1452(5) 0.1438(8) y 0.4622(7) 0.4769(4) 0.4774(9) 0.4654(8) 0.7450(4) 0.4529(7)

53 z 0.232(6) 0.2292(3) 0.2305(6) 0.2326(7) 0.4163(3) 0.2244(7)

U(eq) 0.0318(2) 0.0305(1) 0.0317(3) 0.0370(3) 0.0202(1) 0.028(3) Occupancy 1 1 1 1 1 1 O(3) x 0.1209(7) 0.1205(3) 0.1203(7) 0.1206(7) 0.1225(4) 0.1293(8) y 0.7443(6) 0.7514(3) 0.7522(8) 0.7424(7) 0.7845(4) 0.7449(7) z 0.4156(5) 0.4139(3) 0.4109(6) 0.4156(6) 0.5964(3) 0.4127(7) U(eq) 0.0262(2) 0.0245(1) 0.0286(3) 0.0243(2) 0.0194(1) 0.026(3) Occupancy 1 1 1 1 1 1 O(4) x 0.4098(6) 0.3964(3) 0.4057(4) 0.4025(8) 0.4057(4) 0.4167(7) y 0.7847(5) 0.7804(3) 0.7779(8) 0.7837(7) 0.5601(4) 0.7849(7) z 0.5968(5) 0.6015(2) 0.6012(6) 0.5983(6) 0.2511(4) 0.5914(7) U(eq) 0.0191(2) 0.0183(1) 0.0287(2) 0.0209(2) 0.0228(1) 0.022(3) Occupancy 1 1 1 1 1 1 O(5) x 0.3678(7) 0.3692(4) 0.3693(7) 0.3715(8) 0.3694(5) 0.3642(8)

TABLE 5.2 (continued) y 0.561(6) 0.5517(3) 0.5483(8) 0.5586(8) 0.5489(5) 0.5633(7) z 0.2511(5) 0.2597(3) 0.2562(6) 0.2508(7) 0.529(4) 0.2359(7) U(eq) 0.0242(2) 0.0253(1) 0.0236(2) 0.0266(2) 0.0266(1) 0.026(3) Occupancy 1 1 1 1 1 1 O(6) x 0.1391(7) 0.1372(4) 0.1368(7) 0.1415(9) 0.1407(5) 0.1416(7) y 0.5483(6) 0.5388(3) 0.5341(8) 0.5477(9) 0.7463(4) 0.5579(7) z 0.5295(5) 0.5383(3) 0.5368(6) 0.5317(7) 0.3309(4) 0.5179(7) U(eq) 0.0206(2) 0.0206(1) 0.0213(2) 0.0353(3) 0.0261(1) 0.023(3) Occupancy 1 1 1 1 1 1 54 O(7) x 0.342(7) 0.3487(4) 0.3473(6) 0.3449(7) 0.345(5) 0.3385(8)

y 0.744(5) 0.7302(3) 0.7328(8) 0.7419(7) 0.484(5) 0.7524(7) z 0.3315(5) 0.3469(3) 0.3479(5) 0.3368(6) 0.4895(4) 0.3101(8) U(eq) 0.278(2) 0.0255(1) 0.0189(2) 0.0245(2) 0.0293(1) 0.034(3) Occupancy 1 1 1 1 1 1 O(8) x 0.3699(7) 0.3694(4) 0.3666(8) 0.367(1) 0.3721(5) 0.3692(8) y 0.4841(6) 0.4935(3) 0.496(8) 0.4875(9) 0.9225(4) 0.4724(7) z 0.4895(6) 0.4889(3) 0.4876(6) 0.4898(7) 0.6175(3) 0.4845(7) U(eq) 0.0265(2) 0.0292(1) 0.0279(3) 0.0486(4) 0.0286(2) 0.025(3) Occupancy 1 1 1 1 1 1 O(9) x 0.2517(6) 0.2419(4) 0.2417(6) 0.2491(9) 0.2524(5) 0.2652(7) y 0.9229(5) 0.9185(3) 0.9217(6) 0.9237(8) 0.3617(4) 0.9236(7) z 0.6173(5) 0.6219(2) 0.6212(5) 0.6195(6) 0.4686(3) 0.4591(7) U(eq) 0.0184(1) 0.0217(1) 0.0124(1) 0.0327(3) 0.0187(1) 0.019(3)

TABLE 5.2 (continued) Occupancy 1 1 1 1 1 1 O(10) x 0.2082(7) 0.2113(4) 0.2137(7) 0.2085(9) 0.2053(5) 0.2028(8) y 0.3619(5) 0.3613(3) 0.3604(6) 0.3638(7) 0.6736(4) 0.3615(6) z 0.4679(5) 0.4716(2) 0.4705(5) 0.4705(6) 0.4881(4) 0.4591(7) U(eq) 0.0196(2) 0.0172(1) 0.0133(2) 0.0225(2) 0.0398(2) 0/019(3) Occupancy 1 1 1 1 1 1 O(11) x 0.2928(7) 0.2737(4) 0.273(7) 0.2903(1) 0.2966(6) 0.3125(7) y 0.6751(6) 0.6741(3) 0.6734(6) 0.6729(7) 0.5634(4) 0.6719(6) z 0.4885(5) 0.4984(2) 0.4989(5) 0.4909(7) 0.3726(3) 0.4737(6)

55 U(eq) 0.0275(2) 0.0245(2) 0.0181(2) 0.042(4) 0.0179(1) 0.016(3)

Occupancy 1 1 1 1 1 1 O(12) x 0.2144(6) 0.2116(3) 0.2141(6) 0.2147(8) 0.2123(5) 0.2152(7) y 0.5628(5) 0.5622(3) 0.5627(6) 0.5635(6) 0.6005(4) 0.5667(7) z 0.3722(4) 0.3782(2) 0.3764(5) 0.3747(5) 0.6387(3) 0.3635(7) U(eq) 0.014(1) 0.0151(1) 0.0163(2) 0.0209(2) 0.0169(1) 0.021(3) Occupancy 1 1 1 1 1 1 O(13) x 0.3117(6) 0.3086(3) 0.3058(6) 0.3121(8) 0.3123(4) 0.3121(7) y 0.6015(5) 0.6001(3) 0.6004(6) 0.603(6) 0.6005(4) 0.6007(7) z 0.6385(5) 0.6456(2) 0.6459(5) 0.6412(6) 0.6387(3) 0.6270(6) U(eq) 0.0173(1) 0.0152(1) 0.0133(2) 0.0198(2) 0.0169(1) 0.019(3) Occupancy 1 1 1 1 1 1 O(14) x 0.1883(6) 0.1936(3) 0.1921(6) 0.1893(7) 0.1869(4) 0.1846(7) y 0.6854(5) 0.6859(3) 0.6845(6) 0.684(7) 0.683(4) 0.6852(7)

TABLE 5.2 (continued) z 0.2439(4) 0.2487(2) 0.2491(6) 0.2449(6) 0.2435(3) 0.2342(7) U(eq) 0.0148(1) 0.0153(1) 0.0174(2) 0.0176(2) 0.0169(1) 0.022(3) Occupancy 1 1 1 1 1 1 O(15) x 0.1894(6) 0.2137(3) 0.2108(6) 0.1907(8) 0.188(4) 0.1863(7) y 0.8851(5) 0.8797(3) 0.8793(7) 0.8841(7) 0.8848(4) 0.8888(6) z 0.3128(5) 0.3137(2) 0.3133(5) 0.3156(6) 0.3135(3) 0.3066(6) U(eq) 0.0164(2) 0.0138(1) 0.0154(2) 0.022(2) 0.018(1) 0.018(3) Occupancy 1 1 1 1 1 1 O(16A) x 0 0 0 0 0 0

56 y 0.7528(1) 0.7229(3) 0.7376(6) 0.7546(5) 0.7506(2) 0.7609(12)

z 0.25 0.25 0.25 0.25 0.25 0.25 U(eq) 0.04 0.04 0.04 0.04 0.04 0.049(5) Occupancy 0.30(1) 0.20(2) 0.16(2) 0.13(1) 0.191) O(16B) x 0 0 0 0 0 y 0.6592(2) 0.6813(2) 0.6834(3) 0.6619(1) 0.6543(1) z 0.25 0.25 0.25 0.25 0.25 U(eq) 0.04 0.04 0.04 0.04 0.04 Occupancy 0.24(1) 0.23(2) 0.30(2) 0.37(1) 0.24(1) O(17) x 0.4994(1) 0.4924(6) 0.4953(1) 0.5008(1) 0.5004(8) 0.5015(9) y 0.4181(1) 0.4026(5) 0.4039(9) 0.4049(1) 0.4164(8) 0.4409(9) z 0.1442(9) 0.1551(5) 0.1489(7) 0.145(1) 0.1427(6) 0.1386(9) U(eq) 0.04 0.04 0.04 0.04 0.04 0.048(4) Occupancy 0.78(2) 0.78(1) 0.98(2) 0.75(2) 0.68(1)

TABLE 5.2 (continued) O(18) x -0.0196(9) -0.0195(5) -0.0163(9) -0.0183(1) -0.0234(7) -0.0253(10) y 0.6443(8) 0.6642(4) 0.6641(9) 0.6485(1) 0.6449(6) 0.6223(10) z 0.5691(7) 0.5530(4) 0.5496(7) 0.5637(9) 0.5705(6) 0.5870(10) U(eq) 0.04 0.04 0.04 0.04 0.04 0.060(4) Occupancy 0.94(2) 0.97(1) 1 0.87(2) 0.80(1) O(19) x 0.0268(1) 0.0272(5) 0.0233(8) 0.0264(9) 0.0294(7) 0.0314(9) y 0.5492(9) 0.5486(4) 0.548(9) 0.5449(1) 0.5496(7) 0.5621(8) z 0.3679(8) 0.3775(4) 0.3758(7) 0.3768(9) 0.3701(6) 0.3391(8) U(eq) 0.04 0.04 0.04 0.04 0.04 0.046(4)

57 Occupancy 0.81(2) 0.93(1) 0.99(2) 0.89(2) 0.78(1)

O(20) x 0.4972(1) 0.5067(7) 0.5061(1) 0.4983(1) 0.4959(1) -0.119(11) y 0.6469(1) 0.6224(6) 0.6206(1) 0.6369(1) 0.6404(9) 0.8529(11) z 0.4243(1) 0.401(6) 0.402(1) 0.4152(1) 0.4199(8) 0.5792(10) U(eq) 0.04 0.04 0.04 0.04 0.04 0.081(5) Occupancy 0.63(2) 0.67(1) 0.70(2) 0.61(2) 0.60(1) O(21) x 0.5025(3) 0.5025(1) 0.5011(1) 0.5064(1) 0.5099(2) 0.517(3) y 0.6717(3) 0.6593(6) 0.6628(1) 0.6764(1) 0.6842(2) 0.758(2) z 0.2946(2) 0.2756(6) 0.2839(1) 0.2832(1) 0.2759(2) 0.287(2) U(eq) 0.04000(0) 0.04 0.04 0.04 0.04 0.096(13) Occupancy 0.24(2) 0.55(1) 0.61(2) 0.58(2) 0.24(1) O(22) x 0.5 0.5 0.5 0.5 0.5 0.50000(0) y 0.79(2) 0.82(7) 0.82(1) 0.82(2) 0.81(1) z 0.25 0.25 0.25 0.25 0.25

TABLE 5.2 (continued) U(eq) 0.04 0.04 0.04 0.04 0.04 Occupancy 0.26(1) 0.39(7) 0.42(1) 0.33(1) 0.25(1) ______Symmetry transformations used to generate equivalent atoms: #1 -x+1/2,y+1/2,z #2 -x+1/2,-y+3/2,z+1/2 #3 -x+1/2,-y+3/2,z-1/2 #4 -x+1/2,y-1/2,z #5 x,-y+1,z-1/2 #6 -x+1,y,-z+1/2 #7 x,-y+1,z+1/2 #8 -x,-y+1,-z+1 #9 x+1/2,-y+3/2,-z+1 #10 x-1/2,-y+3/2,-z+1 #11 -x,y,-z+3/2 Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (Å2x 103) Na-MS. U(eq) is defined as one third of the trace of the orthogonalized

58 Uij tensor.

The structures of natural metaschoepite, Na-MS-AB1, Na-MS-AB2, and Na-MS-

CRY-Np were refined using the structure of Na-MS-CRY as the starting model. Data quality varied for these crystals, and in some cases the data did not support refinement of anisotropic displacement parameters for the O sites. Otherwise, the refinement strategy for these crystals was the same as for Na-MS-CRY, and the occupancies of the O16 through O22 sites were refined with their corresponding displacement parameters fixed.

Crystallographic parameters for each are given in Table 5.1. Atomic parameters are in

Table 5.2. Selected interatomic distances for the five refined structures, as well as those of Weller et al. (2000) for synthetic metaschoepite, are compared in Table 5.3.

5.3.3 Structure description

The structures of each crystal under study contain four symmetrically distinct U6+ cations, and each are strongly bonded to two atoms of O, resulting in approximately

2+ linear (UO2) uranyl ions. The uranyl ions are each coordinated by a total of five O atoms and hydroxyl groups, and these ligands are arranged at the equatorial vertices of pentagonal bipyramids that are capped by the O atoms of the uranyl ions (Table 5.3).

The uranyl pentagonal bipyramids are linked by sharing equatorial edges, which results in sheets that are topologically identical to those found in the structures of metaschoepite, schoepite, and fourmarierite (Figure 5.3). However, examination of the bond-strength distributions within the uranyl polyhedra reveals that the composition of the sheets in the structures under study are not the same as that of the electroneutral sheets in metaschoepite and schoepite.

TABLE 5.3 SELECT INTERATOMIC BOND LENGTHS DISTANCES FOR Na METASCHOEPITE FOR THE FIVE REFINED STRUCTURES, AS WELL AS THOSE OF WELLER ET AL. (2000) ______

Natural MS Na-MS-CRY Na-MS-CRY-Np Na-MS-AB1 Na-MS-AB2 Weller ______U(1) – O7 1.745 (10) 1.770 (5) 1.731 (9) 1.747 (11) 1.781 (7) 1.766 (13) O3 1.796 (10) 1.780 (5) 1.767 (10) 1.788 (10) 1.777 (6) 1.754 (12) O11 2.302 (9) 2.219 (4) 2.238 (9) 2.281 (12) 2.323 (6) 2.416 (10)

60 O15 2.328 (8) 2.298 (4) 2.291 (10) 2.325 (11) 2.327 (6) 2.303 (9)

O10 2.426 (8) 2.395 (4) 2.366 (9) 2.447 (10) 2.445 (6) 2.445 (10) O14 2.430 (8) 2.439 (4) 2.433 (10) 2.452 (10) 2.453 (6) 2.433 (11) O12 2.558 (7) 2.524 (4) 2.513 (9) 2.527 (9) 2.552 (5) 2.540 (9) U(2) – O1 1.693 (11) 1.764 (6) 1.648 (11) 1.698 (14) 1.767 (7) 1.737 (13) O4 1.829 (9) 1.769 (5) 1.753 (11) 1.754 (11) 1.767 (7) 1.777 (11) O9 2.327 (7) 2.242 (4) 2.280 (9) 2.249 (11) 2.230 (6) 2.208 (9) O11 2.325 (7) 2.253 (4) 2.242 (8) 2.323 (12) 2.344 (6) 2.434 (10) O13 2.443 (7) 2.423 (4) 2.417 (9) 2.432 (9) 2.461 (5) 2.446 (9) O14 2.467 (8) 2.458 (4) 2.484 (10) 2.469 (10) 2.473 (6) 2.452 (11) O10 2.615 (8) 2.691 (4) 2.685 (8) 2.609 (10) 2.605 (5) 2.578 (10) U(3) – O2 1.741 (11) 1.778 (6) 1.713 (13) 1.693 (15) 1.782 (8) 1.820 (11) O5 1.793 (10) 1.779 (6) 1.761 (10) 1.805 (11) 1.806 (7) 1.773 (11) O9 2.236 (8) 2.224 (4) 2.211 (8) 2.225 (10) 2.235 (6) 2.246 (11)

TABLE 5.3 (continued) O15 2.308 (8) 2.290 (4) 2.300 (10) 2.313 (11) 2.325 (6) 2.315 (10) O12 2.411 (7) 2.426 (4) 2.404 (9) 2.420 (9) 2.430 (5) 2.433 (11) O13 2.461 (8) 2.426 (4) 2.404 (9) 2.461 (9) 2.448 (5) 2.475 (10) O14 2.626 (8) 2.441 (4) 2.552 (9) 2.603 (10) 2.605 (6) 2.636 (10)

U(4) – O8 1.774 (11) 1.789 (5) 1.738 (12) 1.709 (17) 1.793 (8) 1.810 (11)

O6 1.796 (10) 1.781 (6) 1.768 (11) 1.767 (14) 1.781 (8) 1.784 (11)

O9 2.222 (8) 2.249 (4) 2.222 (8) 2.227 (10) 2.223 (6) 2.207 (10)

O11 2.309 (9) 2.246 (4) 2.230 (9) 2.249 (11) 2.304 (6) 2.403 (9)

61 O10 2.399 (8) 2.393 (4) 2.395 (9) 2.384 (10) 2.415 (6) 2.378 (10)

O12 2.479 (8) 2.465 (4) 2.475 (9) 2.464 (9) 2.489 (5) 2.481 (11)

O13 2.574 (8) 2.591 (4) 2.593 (9) 2.584 (10) 2,566 (6) 2.565 (11)

O(11) – U4 2.309 (9) 2.246 (4) 2.230 (9) 2.249 (11) 2.304 (6)

U1 2.302 (9) 2.219 (4) 2.238 (9) 2.281 (12) 2.323 (6)

U2 2.325 (9) 2.253 (4) 2.242 (9) 2.323 (12) 2.344 (6)

Figure 5.3 Polyhedral representation of the structure of Na substituted metaschoepite.

Consider the structure of metaschoepite (Weller et al. 2000). Atoms O1 through O8 are part of uranyl ions. Atoms O9 through O15 are all at equatorial vertices of the uranyl pentagonal bipyramids, and O9 through O14 are each

bonded to three U6+ cations, whereas O15 is bonded to two. The bond valences, calculated using the parameters of Burns et al. (1997b), incident upon the O10 through O15 sites due to the bonds to U6+ range from 1.19 to 1.44 v.u., all of which are consistent with occupancy of these sites by OH groups. The bond- valence sum at the O9 site, which forms substantially shorter bonds with U6+, is

2.12 v.u., indicating that the site must be occupied by O. Compare to these values those from the structure of Na-MS-CRY. The O9 and O11 sites both have incident bond-valences of 2.05 v.u., which indicates both contain O only, in contrast to metaschoepite, for which the O11 site contains OH. The remaining sites in the range O10 to O15 in Na-MS-CRY have incident bond-valences in the range of 1.23 to 1.30 v.u., which indicates that each of these sites is occupied

mostly by hydroxyl groups. The chemical analysis for crystals of Na-MS-CRY indicated they contain as much as 2 wt. % F, which is presumably distributed over the OH sites.

The sheet of uranyl polyhedra in the structure of Na-MS-CRY has the approximate composition [(UO2)4O2(OH,F)5], which has a net charge of –1. In metaschoepite and schoepite, the sheet composition is [(UO2)4O(OH)6], which is electroneutral, whereas the topologically identical sheet in fourmarierite has composition [(UO2)4O3(OH)4], with a net charge of –2. The composition of the sheet of polyhedra in Na-MS-CRY is intermediate between those of metaschoepite and fourmarierite. The sheet anion-topologies (Burns et al. 1996) corresponding to the sheets in Na-MS-CRY, metaschoepite and fourmarierite are shown in Figure 5.4. Each vertex in the anion topology represents either an O atom or an OH group, and the locations of the OH groups are shown by circles.

The residual charge of the sheet in Na-MS-CRY is balanced by Na located in the interlayer. It is possible that any of the interlayer H2O sites O(16) through

O(22) contain some Na. The O(21) site appears to be most compatible with Na, as it is coordinated by six ligands at distances between 2.16 and 2.99 Å. Three of these are O atoms of uranyl ions located within the sheets on either side, and three are H2O groups that are located in the interlayer. The idealized formula for Na-

MS-CRY is Na[(UO2)4O2(OH)5](H2O)5, neglecting the F that substitutes at OH sites.

The structures of each of the crystals Na-MS-CRY-Np, Na-MS-AB1, Na-

MS-AB2 and natural metaschoepite appear to be intermediate between those of

Na-MS-CRY and the structure of synthetic metaschoepite published by Weller et al. (2000). This is most apparent when the bonding environment about the O11 site is considered. In all of the structures it is bonded to three U6+cations. In the structure of Weller et al. (2000), the bond length is 2.42 Å, whereas it is 2.24 Å in Na-MS-CRY. In the other structures, it is 2.24, 2.28, 2.31 and 2.32 for Na-MS-CRY-Np, Na-MS-AB1, natural metaschoepite, and Na-MS-AB2, respectively. The refined occupancies of the O16 through O22 sites also vary amongst these structures, with the highest corresponding to Na-MS-CRY and Na-

MS-CRY-Np and the lowest to Na-MS-AB2.

The unit cell dimensions of the crystals studied, as well as those of synthetic metaschoepite studied by Weller et al. (2000), show significant variations. The situation in these structures is complex, with variations in the amount of OH at the O11 site, partial occupancies of the H2O sites in the

interlayer and the associated variability of their H bonding networks, and the substitution of Na for H2O in the interlayer. Despite this, there is a strong linear relationship between the bond-valence sum incident upon the O11 site and the b unit cell dimension (Figure 5.5). The bond-valence sum at the site reflects the quantity of OH at the site. The higher the OH content of the O11 site, the larger

the average O11- U6+ bond length, and the lower the bond-valence sum incident upon the O11 site. As OH enters the O11 site, the b repeat distance steadily decreases. If the size of the O11 site was the only factor, one would expect the opposite to occur. Thus, the decrease of the b dimension is presumably related to distortions of the sheet of uranyl polyhedra that facilitate enhanced H bonding from the O11 site to an interlayer H2O group. Stronger H bonding might tend to

pull the O11 atom further out of the plane of the U6+ cations (towards the interlayer), perhaps increasing the corrugation of the sheet, which would tend to decrease the b cell dimension.

Figure 5.5 Bond valence sums at O11 for all Na metaschoepite structures against corresponding b unit cell dimensions.

5.4 Np5+ incorporation into Na-MS-CRY-Np

The interaction of groundwater with spent nuclear fuel in a geological repository within the unsaturated zone, such as the proposed repository at Yucca

Mountain, would likely entail the formation of a suite of uranyl minerals (e.g.,

Wronkiewicz et al. 1992, 1996, Finn et al. 1996, Finch et al. 1999). At the onset of alteration, temperatures may still be significantly elevated above ambient conditions. Simulations using spent nuclear fuel in contact with small amounts of water EJ-13 from the Yucca Mountain site resulted in the formation of a phase identified as metaschoepite (Finch et al. 1999). Given the relatively high abundance of Na in EJ-13 water, as well as in the groundwater associated with

Yucca Mountain, it is possible that Na-MS formed in some of the simulations, and that it might be an important phase in the repository environment.

Burns et al. (2004) examined incorporation of Np5+ in powders of uranyl compounds synthesized under mild hydrothermal conditions. They found no significant incorporation in crystals of synthetic metaschoepite that were created by reacting UO with ultrapure water. However, Burns et al. (2004) did find that 3

Np5+ was associated with powders of synthetic uranophane and the Na analogue of compreignacite, and that incorporation into the powders was in proportion to the concentration of Np in the mother solution. Burns et al. (2004) found that

Np5+was associated with powders of minerals that have sheets of uranyl

polyhedra with charges, and interlayer cations, but that Np5+ was not incorporated into powders of the two structures considered that contain electroneutral sheets of

uranyl polyhedra. On this basis, Burns et al. (2004) proposed that the interlayer cations could be involved in the charge-balance mechanism that permitted

+ substitution of Np5+ for U6+.

The sheets of uranyl polyhedra in Na-MS have a residual charge, and the interlayers contain Na. The existence of Na-MS implies that Na can substitute in the interlayer of metaschoepite, which could provide a charge-balance mechanism

for substitution of Np5+ for U6+ within the sheets of uranyl polyhedra. The chemical analysis of crystals of Na-MS-CRY-Np provides compelling evidence that Np is present in the crystal that was synthesized from a solution that was

spiked with Np5+ (Figure 5.2).

No suitable Np standard was available for the ICP-MS study, so it was not possible to quantify the Np concentration in the crystals that were analyzed. Using simply the ratio of the count rates for the 237 (Np) and 238 (U) masses, and the known concentration of U in the crystals, it is possible to estimate that the crystals of Na-MS-CRY-Np contain ~500 ppm Np, which is similar to the concentration in the parent solution.

Incorporation of Np into a single crystal of Na-MS-CRY-Np supports the

observation of Burns et al. (2004) that Np5+ is incorporated into uranyl phases with charged sheets and interlayers. It is especially notable that Burns et al.

(2004) did not find Np incorporated into synthetic metaschoepite that was grown in the absence of Na.

5.5 Discussion

The close structural and chemical relationship of Na-MS to metaschoepite suggests that Na-MS could occur as a mineral. Indeed, the structure determination for a crystal of natural metaschoepite shows that it is intermediate between end- member metaschoepite, as defined by the synthetic crystal studied by Weller et al.

(2000), and end-member Na-MS, which is approximated by Na-MS-CRY. Given that the X-ray powder diffraction patterns of the two phases are essentially indistinguishable, and that only a detailed chemical analysis would reveal the presence of Na, natural occurrences of Na-MS may have been miss-identified as metaschoepite. In a recent study of the thermochemistry of uranyl oxide hydrates,

Kubatko et al. (2006) presented the unexpected result that metaschoepite is likely metastable relative to the oxides under geologically relevant conditions. The degree to which Na substitution in the interlayer of metaschoepite may stabilize the structure is unknown, but this effect could be substantial.

Several synthetic Na uranyl oxide hydrates have been reported, but the structures are only known for three. The Na analogue of compreignacite appears, on the basis of the similarity of their powder diffraction patterns, to be isostructural with compreignacite (Burns 1998). This structure contains sheets of uranyl pentagonal bipyramids that are based upon the protasite (α-U3O8) anion-topology, and Na atoms and H2O groups are located within the interlayer region. The structure of

Na2[(UO2)3O3(OH)2] also contains sheets of uranyl pentagonal bipyramids based upon the protasite anion topology, but in this case only Na exists in the interlayer of the structure (Li and Burns, 2001). The compound Na[(UO ) O (OH) ](H O) 2 4 2 5 2 2

contains an unusual sheet of uranyl pentagonal bipyramids and distorted uranyl square bipyramids, and Na atoms and H2O groups are located in the interlayer

(Burns and Deely, 2002). Na-MS is only the fourth structure known that is based upon sheets of uranyl pentagonal bipyramids with the fourmarierite anion- topology, it possesses a novel sheet composition, and is the first of this structural group that contains Na. This study has demonstrated that the crystals incorporate

Np. This is in contrast to earlier studies that showed no incorporation of Np5+ in synthetic metaschoepite (which has electroneutral sheets), and supports the

hypothesis that Np5+ incorporation is more likely in uranyl oxide hydrates with charged species in the interlayer.

CHAPTER 6:

NEPTUNIUM SUBSTITUTION IN SYNTHETIC URANYL PHASES AS A

FUNCTION OF TEMPERATURE AND pH

Alteration of spent nuclear fuel in a geological repository under oxidizing conditions may result in uranyl compounds. Incorporation of Np-237 into uranyl alteration phases could impact repository performance. Powders of soddyite,

(UO2)2(SiO4)(H2O)2, uranophane, Ca[(UO2)(SiO3OH)]2(H2O)5, becquerelite,

Ca[(UO2)3O2(OH)3]2(H2O)8, Na compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7, and kasolite, Pb[(UO2)(SiO4)](H2O), were synthesized under mild hydrothermal conditions in the presence of Np5+. Powders were synthesized from solutions at various pH and temperatures to investigate how these variables impact the incorporation of Np5+ into each uranyl phase.

Each phase has a limited range of pH values and temperatures under which it can be synthesized with high yield and purity. As many phases are very sensitive to these conditions, only relatively few uranyl phases can be synthesized over wide ranges of either temperature or pH. In the cases of soddyite and becquerelite, a narrow range of pH values produces pure soddyite or becquerelite, but both can be synthesized over a fairly large range of temperatures. Kasolite can be synthesized over a wide temperature range

from solutions with an initial pH ranging from 3.0 to 10.0. Na compreignacite can be synthesized only under limited conditions.

6.1 Synthesis

Synthesis conditions are discussed in Chapter 4, specific synthesis values are listed in

Table 6.1.

6.2 Analysis of samples at ANL

Following cooling of the reaction vessels, samples of the resulting solutions were collected and the solids were recovered by filtration. Crystals of sodium uranyl acetate were present in some of the products, and probably formed as the solutions were cooled or following cooling. Each powder was washed with boiling water to remove these soluble crystals. Subsequently, each powder was divided into three portions; one was used for X-ray powder diffraction analysis, one was prepared for chemical analysis without further treatment, and the third was washed by shaking a combination of the powder and 2 mL of 0.5 M acetic acid for 20 seconds in an attempt to remove adsorbed

Np5+.

Powder diffraction patterns were compared to those provided in the Powder

Diffraction File, as well as those calculated using the corresponding crystal structures and the program ATOMS (Dowty, 2000). There were no detected impurities in the powders under study. Detection of crystalline phases present at levels below  5%, or the presence of amorphous material, is not possible using powder diffraction.

TABLE 6.1 SYNTHESIS CONDITIONS FOR SODDYITE, URANOPHANE, Na COMPREIGNACITE, BECQUERELITE, AND KASOLITE. Soddyite

ID UO2(CH3COO)2 Na2SiO3 H2O Np stock 79mM Acid for pH Final pH Synthesis ·2H2O (g) ·9H2O (g) mL (mL) conc. HCl (mL) Temp. °C S4-80 0.4245 0.1422 5.00 0.0643 0.350 4.06 80 S4-100 0.4245 0.1420 5.00 0.0643 0.610 4.00 100 S4-120 0.4245 0.1422 5.00 0.0643 0.350 3.98 120 S4-140 0.4248 0.1430 5.00 0.0643 0.435 4.09 140 S4-160 0.4242 0.1414 5.00 0.1720 0.520 4.00 160

72 Uranophane

ID UO2(CH3COO)2 Na2SiO3 Ca(CH3COO)2· H2O Np stock Acid for pH Final Synthesis ·2H2O (g) ·9H2O (g) H2O (mL) 79mM (mL) conc. HCl (mL) pH Temp. °C U4-100 0.4249 0.2270 0.2820 5.00 0.0643 0.420 4.02 100 U4-120 0.4249 0.2265 0.2820 5.00 0.0643 0.570 4.00 120 U4-160 0.4247 0.2269 0.2825 5.00 0.0643 0.320 4.00 160 U5-80 0.4249 0.2270 0.2820 5.00 0.0643 1.150 5.09 80 U5-100 0.4248 0.2269 0.2820 5.00 0.0643 1.170 5.03 100 U5-120 0.4248 0.2270 0.2820 5.00 0.0643 0.145 5.01 120 U5-140 0.4249 0.2275 0.2820 5.00 0.0643 0.345 5.12 140 U5-150 0.4208 0.2235 0.2829 5.00 0.0643 0.050 5.05 150 U5-160 0.4249 0.2260 0.2825 5.00 0.0643 0.060 5.05 160 U6-100 0.4244 0.2272 0.2820 5.00 0.0643 0.000 6.10 100 U6-120 0.4244 0.2272 0.2820 5.00 0.0643 0.030 6.00 120 U6-160 0.4249 0.2230 0.2824 5.00 0.0643 0.060 5.99 160

TABLE 6.1 (continued)

Becquerelite Na compreignacite ID ID UO2(CH3C Na2SiO3 H2O (mL) Np stock Acid for pH Base for Final pH Synthesis OO)2 ·9H2O (g) 79mM (mL) conc. HCl pH 10% Temp. °C ·2H2O (g) (mL) NaOH (mL) B5-80 NC5.5-100 0.4248 0.040 5.00 0.143 0.050 NA 5.48 100 B5-100 NC6-100 0.4245 0.045 5.00 0.143 0.035 0.020 5.96 100 B5-120 NC7-100 0.4355 0.039 5.00 0.143 NA 0.010 7.03 100 B5-150 NC5.5-120 0.4250 0.041 5.00 0.143 0.020 NA 5.53 120 NC6.5-120 0.4253 0.041 5.00 0.143 0.010 0.080 6.49 120

Kasolite UO (CH COO) Pb(CH COO) Na SiO H O Np stock Acid for pH Base for pH Final Synthesis ID 2 3 2 3 2 2 3 2 ·2H2O (g) ·3H2O (g) ·9H2O (g) (mL) (mL) 12M HCl (mL) conc. Amm. pH Temp. °C K-80 0.426 0.2747 0.2484 5.0262 0.13 5.027 80 K-100 0.4208 0.2765 0.2508 4.9866 0.13 4.868 100 K-120 0.4243 0.2695 0.2559 5.0175 0.13 4.849 120 K-150 0.4282 0.2754 0.2544 5.0175 0.13 5.103 150 K-4 0.4271 0.2753 0.258 4.9601 0.06 0.25 0.01 2.900 150 K-5 0.4218 0.2724 0.2551 4.9560 0.06 0.12 3.770 150 K-6 0.428 0.2695 0.252 4.9637 0.06 0.01 5.130 150 K-7 0.4249 0.2744 0.2547 5.2493 0.06 0.01 0.12 5.870 150 K-8 0.4219 0.2757 0.2553 4.9962 0.06 0.06 0.24 7.440 150 K-9 0.4251 0.2745 0.2542 5.0724 0.06 0.08 0.34 8.480 150 K-10 0.4229 0.2744 0.2495 5.0247 0.06 0.10 0.24 9.330 150 NA indicates sample was not available for analyses

Chemical analyses were done for aliquots of solutions collected prior to hydrothermal treatment, precipitates that were washed with boiling water and precipitates that were washed using both boiling water and acetic acid. In most cases precipitates had formed in the solutions prior to their analysis. The samples were acidified by adding 1 mL of 1 M HCl, which further dissolved the precipitates. Some of the solutions contained white gel-like amorphous silica even after acidification. Powders for analysis were each dissolved in 5 mL of 0.6 M hydrochloric acid.

6.3 Analysis of samples synthesized at the University of Notre Dame

Following cooling of the reaction vessels, samples of the resulting solutions were collected and the solids were recovered by centrifuging the samples and removing the solution, thereby preserving the maximum amount of sample. Contaminant precipitates of sodium uranyl acetate (identified by X-ray crystallography) were present in some of the products, and probably formed as the solutions were cooled or following cooling. Each powder was washed with water repeatedly to remove these soluble crystals.

Subsequently, each powder was divided into three portions; one was used for X-ray powder diffraction analysis, one was prepared for chemical analysis without further treatment, and the third was washed by shaking a combination of the powder and 3 mL of

0.5 M acetic acid for 5 minutes (or until the solution had a light yellow color indicating partial dissolution of each powder) in an attempt to remove adsorbed Np5+. In the case of becquerelite, the powders were washed with 3 mL of 0.2 M acetic acid and shaken until partial dissolution occurred; a weaker acid solution was used due to high dissolution rates in the 0.5 M acetic acid solution.

Chemical analyses were performed for precipitates that were washed with boiling water, and precipitates that were washed using both boiling water and acetic acid.

Powders for analysis were each dissolved in 10 mL of 1 M hydrochloric acid and then further diluted for ICP analysis with 15-20 mL of 1 M hydrochloric acid

6.4 Results

The results of the chemical analyses for powders of synthetic soddyite, uranophane, and Na-compreignacite conducted at Argonne National Lab are summarized in Table 6.1. Considering the analyses of U for soddyite, uranophane, becquerelite, kasolite and Na-compreignacite, it is apparent that most of the analyses gave a value differing from the theoretical concentration of U in the synthetic mineral. This bias in the analyses probably arises from our inability to separate very fine-grained powders of the solid phases from fibers of the paper filters used to recover the powders. The powders were removed from the filters by scraping, which invariably resulted in microscopic fragments of the filter paper being included in the specimen for chemical analysis.

Following weighing of the powder (and filter paper fragments), the powders were dissolved, and visual inspection confirmed the presence of filter paper fragments in the solutions. As such, the mass of powder we measured is an upper bound to the total mass of the target mineral analyzed. The ratios of Np and U from the analyses should be the same as those in the synthesized phase, despite being systematically low because of the presence of fibers of filter paper. In the discussion that follows, emphasis is therefore placed upon the ratio of the concentration of Np to that of U. Note also that the Np

content of the parent solution of each experiment prior to heating varied from 136 to 676 ppm (Table 6.2).

Where data are compared in the following figures, the Np concentrations of the synthetic powders have been normalized to an initial solution concentration of 250 ppm.

Both the measured and normalized values are given in Table 6.1. Burns et al. (2004) found that the Np concentration in the Na analogue of compreignacite and uranophane synthesized under similar conditions varied linearly with the concentration of Np in the initial solution (Figure 6.1).

600

Uranophane 500 Na Compreignacite Beta (UO2)(OH)2 400 Meta Schoepite

300

200

100

0 Np Content of Crystals (ppm of total U+Np) total of (ppm Crystals Npof Content 0 0.1 0.2 0.3 0.4 0.5 0.6 Np in Initial Solution (mg/ml)

Figure 6.1 Burns et al. (2004) results for Np incorporation into uranophane, Na

compreignacite, beta UO2(OH)2, and metaschoepite, with Np ppm of the total actinides on the Y axis and the initial quantity of Np5+ in the mother solution on the X axis.

6.4.1 Results for Soddyite

The analyses in Table 6.2 indicate that Np was detected in all of the powders of soddyite analyzed, as determined by ICP-MS with a minimum detection limit of 1 ppb.

There are significant differences between the Np concentrations across the series of samples. Consider first the case of soddyite synthesized from a solution with a starting pH of 4.0. The lowest concentration of Np occurs in the sample grown at 80°C, and is approximately 114 ppm (normalized to 210 ppm). The concentration of Np steadily increases over the temperature range 80 to 160 °C with a concentration of 1619 ppm

(normalized to 599 ppm) for the sample grown at 160°C, and is close to linear within the accuracy of the data (Figure 6.2). There is good correspondence between the Np concentration for powders washed with water and those washed with acetic acid, suggesting that the Np is largely present in a solid phase, rather than sorbed onto the surface of grains. It is not possible to eliminate the possibility that a Np-rich phase is present in the samples, as such a phase would be undetectable by X-ray powder diffraction.

Efurd et al. (1998) provided calculated solubility curves for Np2O5 as well as

NpO2(OH)(am) and NaNpO2CO3.3.5H2O at 25°C in water with various dissolved ions

(Figure 6.3). According to these solubility curves, for a solution with a pH of 4.0 and a

Np concentration of 0.001 M, no Np-rich precipitate would be expected at room temperature.

TABLE 6.2 SYNTHESIS CONDITIONS AND ANALYTICAL RESULTS FOR POWDERS OF URANOPHANE, SODDYITE, AND Na COMPREIGNACITE SYNTHESIZED AT ANL Np concentration ppm Np normalized to 250 ppm in initial solution T pH U (wt%) U (wt%) Np in Np (ppm) Np (ppm) Np (ppm) Np (ppm) (C) water acid initial water acid water acid washed washed solution washed washed washed washed (ppm) powder powder powder powder Soddyite S4-80 80 4 53.75 64.88 136 114 137 210 252

78 S4-100 100 4 69.09 66.58 166 300 295 452 444

S4-120 120 4 65.06 66.39 195 379 381 486 488 S4-140 140 4 67.44 68.06 178 460 462 646 649 S4-160 160 4 61.22 60.66 676 1619 1552 599 574 Theoretical 71.25 71.25 Uranophane U4-100 100 4 51.33 44.27 187 51 42 68 56 U4-120 120 4 40.59 38.32 171 29 15 42 22 U4-160 160 4 42.98 31.61 239 17 26 18 27 U5-80 80 5 48.01 42.07 162 87 82 134 127 U5-100 100 5 31.67 47.62 324 66 91 51 70 U5-120 120 5 50.68 47.69 202 73 65 90 80 U5-140 140 5 51.07 41.74 184 48 42 65 57 U5-160 160 5 50.70 47.73 123 133 142 270 289 U6-100 100 6 54.05 53.47 174 164 161 236 231 U6-120 120 6 52.66 55.71 193 100 94 130 122 U6-160 160 6 40.17 42.13 241 71 84 74 87 Theoretical 55.6 55.6

TABLE 6.2 (continued) Np concentration ppm Np normalized to 250 ppm in initial solution T pH U (wt%) U (wt%) Np in Np (ppm) Np (ppm) Np (ppm) Np (ppm) (C) water acid initial water acid water acid washed washed solution washed washed washed washed (ppm) powder powder powder powder Na Compreignacite N5.5-100 100 5.5 71.54 69.73 581 723 736 256 260 N6-100 100 6 68.92 NA 576 536 NA 220 NA N7-100 100 7 45.81 68.04 577 251 199 99 78 N5.5-120 120 5.5 67.96 NA 580 268 NA 104 NA

79 N6.5-120 120 6.5 71.99 NA 523 859 NA 376 NA

theoretical 71.11 71.11

NA : indicates sample was not analyzed

Figure 6.2 Results of analyses for powders of soddyite versus synthesis temperature. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using 0.5 M acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution.

The crystal chemistry of Np5+ is similar to that of U6+ in that nearly linear actinyl ions dominate in each case, and the neptunyl and uranyl ions are of similar size (Chapter

1). Neptunyl ion incorporation into the structure of soddyite presumably occurs by substitution for uranyl ions, as there are no interstitial cations and the Si4+ sites are highly incompatible with the geometrical requirements of Np5+. Substitution of Np5+ for U6+ requires a charge-balancing substitution elsewhere in the crystal structure. The possibilities are limited in soddyite to the Si4+ site, one of the O sites, the site that contains H2O, or an interstitial site that is normally vacant.

A.)

B.)

Figure 6.3. A.) Calculated solubility curves for Np2O5, NpO2(OH) and

NaNpO2CO3·3.5H2O in J-13 water at 25˚C (discussed in Chapter 1). B.)

Calculated solubility curves for Np2O5, NpO2(OH) and NaNpO2CO3·3.5H2O in UE-25#1(carbonate rich ground water from the Yucca Mountain site) water at

25˚C. The dashed lines indicate the solubility of Np2O5 using the value from Lemire (1984). (from Kaszuba and Runde, 1999)

In the case of Si4+, substitution of P5+ would provide charge balance and would potentially alleviate some structural strain associated with local substitution of the slightly larger Np5+ cation for U6+ (Forbes and Burns, 2006). Phosphorous was not added to the synthesis, but trace levels may have been present in reagents used in the reactions.

Substitution of hydroxyl at either of the crystallographic O sites seems unlikely. One of these corresponds to the O atoms of the uranyl ions, and OH has never been found as part of a uranyl ion. The other O atom site is an equatorial vertex shared between two uranyl polyhedra that is also shared with a silicate tetrahedron. Incorporation of OH at this site would result in substantial over-bonding at the O site.

The most plausible charge-balancing mechanism for Np5+ incorporation in soddyite may be the occurrence of Na in an interstitial position. There are relatively large channels that extend along [110] in the crystal structure that are bounded by O atoms of uranyl ions as well as H2O groups that are equatorial vertices of uranyl polyhedra on either side of the channel (Figure 6.4). The minimum distance between the bounding O atoms is 3.8 Å. The special position at 5/8, 1/8, 1/8 is located in this channel and is vacant, and location of a Na cation here would place it 2.42 Å from four O atoms of uranyl ions. This site is 3.23, 3.80 and 4.18 Å from the nearest H, U, and Si atoms, respectively. The use of sodium metasilicate in the synthesis procedure provided Na greatly in excess of the quantity required to balance the charge associated with incorporation of Np.

The continuous increase of Np incorporation in powders of soddyite with synthesis temperature is consistent with incorporation by substitution at the U6+ site.

Such substitution will introduce a degree of structural strain due to the different sizes of

the Np5+ and U6+ cations, as well as the different bonding requirements of their respective actinyl ions.

Figure 6.4 The structure of soddyite structure showing the open cavities that form channels.

Incorporation of Na in interstitial voids to provide charge balance will also create structural strain. Elevated temperature should partially alleviate strain associated with Np incorporation, thus the steady increase of Np incorporation with temperature is commensurate with expectations.

6.4.2 Results for Uranophane

Uranophane was synthesized over the temperature range of 80-160˚C and a pH range of 4.0-6.0. The data for the case of uranophane synthesized from solutions with an initial pH of 5 (Figure 6.5) will be discussed first. Considering first powders synthesized in the range of 80 to 140°C, those grown at 80°C contain the greatest quantity of Np

(~166 ppm, normalized), although even this value is less than the Np content of any of the synthetic soddyite powders (see Table 6.2). The Np content of the powders decreases by ~50% from 80°C to 140°C, and the powders washed using acid contain comparable

amounts of Np as those that were only washed in H2O. The powder synthesized at 160°C contains about 400% more Np than that at 140°C.

As was the case for soddyite, an increase of the temperature of the synthesis might be expected to facilitate incorporation of more Np, not less as observed. The dramatically higher concentration of Np in the powder synthesized at 160°C is inconsistent with the trends of Np concentration in powders synthesized over the temperature range 80 to 140°C.

Figure 6.5 Results of analyses of powders of uranophane synthesized from a solution with an initial pH of 5 versus synthesis temperature. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution (see text for details).

The Np concentrations in powders of uranophane synthesized from solutions with initial pH values of 4.0 and 6.0 (Table 6.2, Figures 6.6 and 6.7) are broadly consistent

with those grown from a solution with a pH of 5.0. In both cases the Np content of the powder decreased with the synthesis temperatures.

Figure 6.6 Results of analyses of powders of uranophane synthesized from a solution with an initial pH of 4.0 versus synthesis temperature. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution.

Powders of uranophane prepared at 100 and 120°C from initial solutions with a

pH of 4.0, 5.0 and 6.0 show an increase in the Np content of the powder with the pH

of the initial solution shown in Figure 6.8 for the case of 100˚C. If a single Np rich

phase is responsible for the bulk of the Np found in the samples of uranophane

synthesized under various conditions, the solubility of that phase apparently decreases

with increasing pH.

Figure 6.7 Results of analyses for powders of uranophane synthesized from a solution with an initial pH of 6.0 versus synthesis temperature. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution.

According to the calculated solubility curves provided by Efurd et al. (1998), this is the case for both Np2O5 and NpO2(OH) over the pH range in question. Friese et al.

(2006) also found higher levels of Np associated with synthetic uranophane grown under alkaline conditions, which possibly result from a Np-rich minor phase. However it is also possible that Np is incorporated into uranophane in low quantities, perhaps due to the lack of an adequate charge balance mechanism.

Figure 6.8 Results for powders of uranophane synthesized at 100˚C with an initial mother solution pH of 4.0, 5.0, and 6.0 shown in red, green and blue respectively. Squares represent data for samples washed with hot water, whereas diamonds are for powders that were also washed using acetic acid. Data have been normalized on the basis of the Np concentration of the initial solution.

6.4.3 Results for Na compreignacite

Na compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7 , was only produced purely at the solution pH values of 5.5, 6.0, and 7.0 and temperatures of 100˚C and 120˚C (Figures 6.9,

6.10, and 6.11 and summarized in Table 6.2). The initial Np concentration in the mother solutions ranged from 523 ppm to 581 ppm. The limited data available indicate a substantial decline in the concentration of Np in the synthesis of powders with increasing pH of the initial solutions. This is in contrast to the results for uranophane. In the case of

Na compreignacite, the Np content of the powders decreases by a factor in excess of 3 over the synthesis pH range of 5.5 to 7.0. There is close agreement between the analysis of water washed and acid washed samples, which indicates little or no adsorption of Np

was removed by the acid wash process. The level of incorporation is similar to that seen in the case of uranophane; at its maximum Np incorporation is only approximately similar to the proportion of Np present in the mother solution.

For Na compreignacite synthesized at 120˚C only two water washed samples (pH of 5.5 and pH of 6.0) were analyzed. In these cases the concentration of Np in the powders increases sharply with increasing pH of the mother solution (Figure 6.10).

The results for Na compreignacite synthesized at an initial pH of 5.5 at 100˚C can be compared to those synthesized at 120˚C. The Np content of powders synthesized at

100˚C following washing with water and acid agree well (723 and 736 ppm respectively).

The water-washed powders synthesized at 120˚C contained 268 ppm Np, a significant decrease relative to the powders grown at 100˚C (Figure 6.11). There is not a sufficient amount of data for Np incorporation into Na compreignacite to be able to determine if Np incorporation is occurring or the details of factors that impact incorporation.

Figure 6.11 Analyses of powders of Na compreignacite synthesized with an initial synthesis pH of 5.5, at 100˚C and 120˚C.

6.4.4 Results for Becquerelite

Becquerelite, Ca[(UO2)3O2(OH)3]2(H2O)8, was synthesized from a solution with an initial pH of 5.0 over the temperature range of 80-150˚C. The synthesis contained 288-

293 ppm of Np5+ (Table 5.3). Results of the chemical analyses of the powders are provided in Table 5.3 and Figure 6.12.

TABLE 6.3

SYNTHESIS CONDITIONS AND ANALYTICAL RESULTS FOR KASOLITE AND BECQUERELITE SYNTHESIZED AT UND

T (C) pH U (wt%) U (wt%) Np in initial Np (ppm) Np (ppm) water washed acid washed solution water washed acid washed (ppm) powder powder Becquerelite B5-80 80 5.23 75.86 72.65 290 344 189 B5-100 100 5.22 72.77 73.61 288 300 195 B5-120 120 5.27 72.03 72.89 289 216 227

91 B5-150 150 5.17 71.88 72.30 293 265 238

theoretical 72.48 72.48 Kasolite K-80 80 5.03 49.58 35.44 267 1251 1273 K-100 100 4.89 39.23 40.26 269 1609 1552 K-120 120 4.85 42.05 40.07 268 1193 1346 K-150 150 5.10 40.74 39.55 267 1555 1437 K4 150 2.90 42.76 35.44 227 96 139 K5 150 3.77 NA 35.44 233 2528 880 K6 150 5.13 34.39 35.44 237 1733 1402 K7 150 5.87 37.76 35.44 222 704 670 K8 150 7.44 41.90 35.44 225 2225 2127 K9 150 8.48 42.39 35.44 218 2769 2487 K10 150 9.33 NA 35.44 223 NA 2832 theoretical 40.53 40.53 NA: Not analyzed

The portions of the powders of becquerelite synthesized at 80˚C and 150˚C that were washed with water have more Np than the corresponding acid-washed powders, which indicates a possible contaminate phase or sorbed Np present. The quantity of Np present in the synthesis is lower than what would be expected to precipitate Np2O5 or

NpO2(OH) (Euford et al. 1998). The lower Np concentration found in some of the acid- washed powders suggests the presence of adsorbed Np.

The data does not indicate any dependence upon temperature. Burns and

Klingensmith (2006) demonstrated incorporation of Np into single crystals of becquerelite, which supports incorporation into the crystal structure. Incorporation of Np may be impacted by other factors over the 70˚C temperature range studied, such as the

Np speciation in the mother solution, for which no data exists.

The divergence in Np concentration in water and acid-washed portions of the powders suggests some Np is either adsorbed on the grains or is contained in an impurity phase. It is important to note that the bulk of the Np remained in solution during the synthesis of becquerelite and only a small proportion was found in the powders.

Alternatively, the becquerelite may have crystallized during heating or cooling, rather than at the maximum temperature of the synthesis. In this scenario, correlation of

Np concentration with maximum heating temperature would not be expected.

6.4.5 Results for Kasolite

6.4.5.1 Np incorporation in Kasolite as a function of pH

Kasolite, Pb[(UO2)(SiO4)](H2O), was synthesized over a wide range of initial solution pH values. To examine the effects of pH on the incorporation of Np5+, kasolite was synthesized with an initial pH of 2.90, 3.77, 5.13, 5.87, 7.44, 8.48, and 9.33, with the syntheses conducted at 150˚C for seven days. Np contained in powders of kasolite, as a function of the initial pH of the synthesis solutions, is shown in Figure 6.13. Powders washed with acid have a similar Np concentration as those washed with water alone. In all cases the powders contained detectable Np. There is a general increase in the Np concentration of the powders with increasing pH, with the exception of the sample

synthesized from a solution with an initial pH of 5.87, which falls significantly below the trend.

6.4.5.2 Np incorporation in powders of Kasolite synthesized at different temperatures

Kasolite was synthesized from solutions with an initial pH of 5.0 at 80, 100, 120, and 150˚C. The results of the analyses of these powders are shown in Figure 6.14. The results do not indicate a strong correlation between Np uptake and maximum synthesis temperature, and are similar for each temperature within expected analytical error. The analyses of the acid-washed and water-washed powders of each synthesis are within error.

6.5 Discussion

The most compelling case for Np5+ incorporation can be made for soddyite. In this case, Np concentrations in the synthetic powders increase rapidly with temperature.

In contrast to soddyite, the data for uranophane reveals less Np incorporation into the synthetic powder, with an increase in incorporation with the pH of the mother solution but not with increasing temperature. These results could suggest that Np is present as a minor phase, but analyses for Np in powders washed in acid and only water agree well, which indicates the solubility of a Np-rich phase, if present, is similar to uranophane.

The data for uranophane are not necessarily inconsistent with Np incorporation into the crystal structure of uranophane, and the trends with temperature and pH may not be related to the charge balance mechanism. There may also be more than one charge balance mechanism in this structure. In the case of uranophane, incorporation of Np may be charge-balanced by incorporation of impurity P5+ for Si4+. A clear similarity between uranophane and kasolite in terms of Np incorporation is the role of pH; the additional

OH- present in higher pH values correlates with increased Np incorporation.

In a study of synthetic single crystals of becquerelite, Burns and Klingensmith

(2006) demonstrated incorporation of Np. The lack of influence of synthesis temperature on incorporation of Np for synthetic powders of becquerelite is similar to that found for

kasolite and uranophane. This could indicate that the charge-balancing mechanism that permits incorporation does not depend upon an expansion of the structure. The lack of

Np incorporation dependence on temperature observed for uranophane, becquerelite and kasolite may relate to the actual crystallization temperature, or the crystallinity of the products. An increase in crystallinity with increasing temperature may decrease Np incorporation possibilities if defects in the micro-crystals play a role in Np incorporation.

The crystallinity of kasolite is shown to increase with increasing synthesis temperature in the powder X-ray diffraction patterns in Figure 6.15. The solids may precipitate at any temperature over the heating cycle, not necessarily at the maximum temperature.

Figure 6.15 Powder X-ray diffraction patterns for kasolite synthesized with Np at 80˚C, 100˚C, 120˚C, and 150˚C.

CHAPTER 7:

Np, P, Mg, Ca, INCORPORATION INTO URANYL PHASES

Studies reported in Chapter 6 examined the impact of synthesis temperature and the pH of the initial mother solution on Np incorporation in uranyl minerals. Charge- balance mechanisms are required for the substitution Np5+ for U6+, and there are several possible mechanisms. The potential presence of a wide range of low level contaminants in the synthesis reactions complicates interpretation of the results. In order to more fully probe charge-balance mechanisms, several syntheses of uranyl phases have been conducted in the presence of Np as well as additional cations.

To minimize structural variations, these experiments emphasized the uranophane- group minerals uranophane, boltwoodite, Na-boltwoodite, and kasolite.

7.1 Experimental

7.1.1 Synthesis

Mild hydrothermal techniques were used for the synthesis of powders of uranophane, kasolite, boltwoodite and Na boltwoodite used in this study. Preliminary experiments in the absence of Np were used for optimization of reaction conditions to increase yield and purity of the products. Reaction conditions were adjusted for maximum purity of synthesis and all experiments for a particular phase were conducted

at the same pH and temperature to minimize variables. Synthesis details are reported in

Chapter 3. A stock solution of Np5+ in 1 M HCl was prepared with a concentration of ~50 mM determined at the University of Notre Dame using ultraviolet-visible spectroscopy.

Synthesis details and conditions are listed in Table 7.1, which also includes information concerning the additional cations that were added.

7.1.2 Chemical Analysis

Following cooling of the reaction vessels, samples of the resulting solutions were collected and the solids were recovered by centrifuging the samples and removing the solutions. Crystals of sodium uranyl acetate were present in some of the products, and probably formed as the solutions were cooled or following cooling. Each powder was washed with water repeatedly to remove these soluble crystals. Subsequently, each powder was divided into three portions; one was used for X-ray powder diffraction analysis, one was prepared for chemical analysis without further treatment, and the third was washed by shaking a combination of the powder and 3 mL of 0.5 M acetic acid for 5 minutes or until the solution had a light yellow color indicating partial dissolution, in an attempt to remove sorbed Np5+.

X-ray powder diffraction patterns were collected for each sample using a Scintag theta-theta diffractometer and CuK radiation. Patterns were collected over the two-theta range 10-90° with a scan rate of 0.5° per minute. The resulting powder diffraction patterns were compared to those provided in the Powder Diffraction File. Samples in which impurities or inaccurate phase formation occurred were discarded and will not be discussed in this chapter. Detection of crystalline phases present at levels below  5%, or the presence of amorphous material, is not possible using powder diffraction.

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TABLE 7.1 SYNTHESIS CONDITIONS FOR URANOPHANE, KASOLITE, Na BOLTWOODITE, AND BOLTWOODITE Uranophane UO (CH COO) Na SiO Ca(CH COO) H O Np stock HPO Acid for Synthesis ID 2 3 2 2 3 3 2 3 Final pH ·2H2O (g) ·9H2O (g) 2·H2O (g) (mL) ~50mM (mL) (g) pH* Temp. C U-150 0.4208 0.2235 0.2829 4.9891 0.13 0.05 5.057 150 UP-150 0.4296 0.2249 0.2830 4.9646 0.13 0.505 0.05 5.069 150 Kasolite

UO2(CH3COO)2 Pb(CH3COO)2 Na2SiO3 Np stock HPO3 Synthesis ID H2O (mL) Final pH ·2H2O (g) ·3H2O (g) ·9H2O (g) ~50mM (mL) (g) Temp. C K-150 0.4282 0.2754 0.2544 5.0175 0.13 5.103 150 KP-150 0.4271 0.2753 0.2580 4.9601 0.13 0.5 5.164 150

101 Na boltwoodite

UO2(CH3COO)2 Na2SiO3 ·9H2O Base for Np stock Synthesis ID H2O (mL) HPO3 (g) Final pH ·2H2O (g) (g) pH* (mL) ~50mM (mL) Temp. C NB 0.1755 0.1408 4.7528 0.25 0.13 10.658 150 NBP 0.1774 0.1468 4.7617 0.2 0.13 0.404 10.513 150 Boltwoodite 6M UO (CH COO) SiO H O Acid for Np stock HPO Final Synthesis ID 2 3 2 2 KOH 2 3 ·2H O (g) (g) (mL) pH* (mL) ~50mM (mL) (g) pH Temp˚C 2 (mL) Bo 0.4249 0.243 0.4946 4.5186 0.13 5.768 150 BoP 0.4266 0.246 0.4982 4.5111 0.13 0.415 5.477 150 UO2(CH3COO)2 6M Acid for Np stock HPO3 MgOH Final Synthesis ID SiO2 H2O ·2H2O KOH pH ~50mM (g) (g) pH Temp˚C BoMg 0.4210 0.2437 0.4905 4.4922 0.05 0.13 0.413 0.5146 5.497 150

UO2(CH3COO)2 6M Acid for Np stock Final Synthesis ID SiO2 H2O CaO (g) ·2H2O KOH pH ~50mM pH Temp˚C BoCa 0.4234 0.2428 0.5038 4.5132 0.04 0.13 0.3089 5.71 150

*Acid for pH adjustment is 12M HCl, all base for pH adjustment is concentrated ammonium hydroxide

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Chemical analyses were performed for powders that were washed with boiling water, and those that were washed using both boiling water and acetic acid. Powders for analysis were each dissolved in 10 mL of 1 M HCl in water and then were further diluted for ICP analysis with 15-20 mL of 1 M HCl. Further details of the analysis are discussed in Chapter 2.

7.2 Results

7.2.1 Results for Uranophane

Earlier studies of uranophane indicated only low concentrations of Np were incorporated into synthetic powders (Chapter 6). Here the impact of added P is examined.

The synthesis temperature selected was 150˚C, as used by Burns et al. (2004) in their study of Np incorporation into uranophane. The quantity of Np present in the mother solution was less than 300 ppm, which at 150˚C is not expected to result in a Np-rich precipitate phase. Phosphorus was added to the mother solution in approximately the same proportion as Np5+.

An additional synthesis experiment was conducted in the absence of added P, but under otherwise identical conditions. The results of this experiment are in accord with previous results for uranophane discussed in Chapter 6 and summarized in Figure 7.1.

Each uranophane powder was synthesized at 150˚C with an initial solution pH of 5.0.

Analyses of the powder of uranophane synthesized in the absence of P gave a Np content of 76 ppm for the powders washed only with water, and 120 ppm when the powders that were also washed in 0.5 M acetic acid. In stark contrast, powders synthesized in the presence of P contained 1450 ppm and 1452 ppm Np for the powders washed only with

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water and with 0.5 M acetic acid, respectively (shown in Figure 7.2). The charge-balance mechanism expected to produce this increased Np incorporation within the structure of uranophane in the presence of P is the substitution of (HPO4) for (SiO3OH).

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7.2.2 Results for kasolite synthesized in the presence of P

Kasolite was synthesized in the presence of added Np and P in similar proportions, as well as without added P (Table 7.2 and Figure 7.3). Each synthesis was conducted at 150˚C with an initial solution pH of 5.0. The Np contents in the synthetic powders following washing with 0.5 M acetic acid were statistically identical for powders synthesized with and without added P. The powders grown in the presence of added P that were washed with hot water contained significantly more Np. It is possible that a relatively soluble Np-phosphate phase precipitated and was removed by the acid- wash protocol. These results indicate that (PO4) substitution for (SiO4) may not be a charge-balancing mechanism in the case of kasolite.

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7.2.3 Results for Na boltwoodite synthesized with added P

Results of chemical analysis for powders of synthetic Na boltwoodite are summarized in Table 7.2. Details of the synthesis are provided in Table 7.1. Two preparations were made, only one of which contained added phosphate. Each was synthesized at 150˚C from solutions with an initial pH of 5.0. The powders synthesized in the absence of added P contain 1430 and 533 ppm, for the water-washed and acid- washed samples respectively. Where phosphate was added, the powders contained 4680 and 1340 ppm for the water-washed and acid-washed portions, respectively

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The presence of Np in a minor precipitate phase seems likely in the case of the

Na-boltwoodite syntheses. Washing with acid removes Np preferentially. The Np contents of the synthetic powders following washing with 0.5 M acetic acid were significantly different, on the order of 3 times for powders synthesized with and without added P. This is similar to the trend observed for uranophane. Despite the uncertainty concerning the presence of a Np-rich precipitate, the data do indicate that phosphate substitution for silicate may be an active charge-balancing mechanism in Na-boltwoodite.

Figure 7.5 Results of analyses for powders of Na boltwoodite synthesized in the presence of Np are shown on the left and powder of Na boltwoodite synthesized in the presence of Np and P are shown on the right. All samples were analyzed in duplicate. Analyses for the water washed samples are shown as red squares and acid washed samples are shown as blue diamonds.

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7.2.4 Results for boltwoodite

The results of analyses of powders of synthetic boltwoodite are given in Table

7.2. Four synthesis experiments were conducted for boltwoodite. All were under identical synthesis conditions, except that P was added to one, Mg and P to another, and

Ca to the final experiment. The experiment in the absence of additional added cations contained 281 ppm Np in the mother solution. The synthesis with P added as the only additional constituent contained 262 ppm of Np and 206 ppm of P. All boltwoodite synthesis experiments were conducted at 150˚C with an initial solution pH of 5.0. The boltwoodite powder grown without additional cations contained 149 and 292 ppm in the water and acid-washed samples respectively. The P experiment had 874 ppm Np in the acid washed sample but 9090 ppm Np in the water washed sample, strongly implying the presence of a Np-rich phase containing phosphate.

Where Mg and P were added to the synthesis, the concentrations for P, Mg, and

Np were 189, 185 and 242 ppm respectively. The water-washed samples for both the P and the Mg experiments contained similar amounts of Np, but whereas in the case of the

P experiment the acid-washed sample has only a fraction of the Np of the water washed sample, the Mg experiment water and acid-washed samples contain a similar amount of

Np. During the acid washing process the grains are partially dissolved, and if a Np rich contaminate phase is present the U/Np ratio would change during washing.

The synthesis with added Ca included 264 ppm of Np and 200 ppm of Ca. The water-washed sample contained 368 ppm of Np and the acid washed sample contained

453 ppm Np. The powders washed using water only and 0.5 M acetic acid contained 368 and 453 ppm Np respectively. In contrast with the experiment conducted in the absence

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of Ca, the addition of Ca did not significantly increase Np incorporation in the synthetic powders.

Figure 7.6 Analyses results for powders of boltwoodite showing all four syntheses, the control experiment with only Np, the P experiment, the Mg experiment and the Ca experiment. All water washed samples are shown in red squares and all acid washed samples are shown in blue diamonds. Every sample was run in duplicate, certain samples are so close as to appear as one.

7.3 Discussion

The results of these experiments demonstrate Np incorporation into powders of uranophane is greatly increased with the presence of P in the mother solution. The charge-balance mechanism that permits Np incorporation in uranophane in the absence of added P appears to accommodate only ~100 ppm Np, but with the addition of P, powders of uranophane incorporate ~1450 ppm Np. In the case of kasolite, the presence of P does

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not appear to impact Np incorporated into synthetic powders. Kasolite appears to have a charge-balance mechanism that can accommodate 1455-1555 ppm Np without additional cations added to the synthesis.

Na boltwoodite showed Np incorporation, 531 ppm Np in the acid washed sample, roughly double the proportion of Np added to the mother solution. The experiment with added P, the acid-washed sample contained about a four-fold increase in

Np incorporation relative to the powder synthesized without added P. This implies that

Na boltwoodite may have a charge balancing mechanism available to incorporate Np, but with the addition of P it has a more compatible charge balancing mechanism available that enables a higher Np content to be incorporated.

In the case of boltwoodite, the powder synthesized without additional cations incorporated Np in proportion to the quantity of Np added in the initial solution, similar to the Np incorporation results for becquerelite in Chapter 6 and Na substituted metaschoepite in Chapter 5. This may indicate that the Na boltwoodite and boltwoodite acid washed samples were not thoroughly rinsed enough to dissolve away all of the sorbed Np, or Np rich phase. The addition of P appears to double the uptake of Np into boltwoodite. The acid-washed samples of the powder synthesized without P added to the powder contained 292 ppm Np, while that synthesized in the presence of P contained 848 ppm. However, when synthesized with the addition of both Mg2+and P, the acid-washed sample contained 4,075 ppm Np. The combination of P5+ and Mg2+ could facilitate a charge-balance mechanism. In the case of Ca added in the mother solution, there appears to be very little or no effect on Np incorporation into the powders of the compounds considered.

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TABLE 7.2

ANALYSIS RESULTS FOR URANOPHANE, KASOLITE, Na BOLTWOODITE AND BOLTWOODITE EXPERIMENTS

T (C) pH U (wt%) U (wt%) Np in initial P in initial Np (ppm) Np (ppm) water acid solution solution water washed acid washed washed washed (ppm) (ppm) powder powder U-150 150 5.057 49.40 83.58 268 0 76 121 UP-150 150 5.069 47.71 46.07 268 257 1452 1454 K-150 150 5.103 40.74 42.93 267 0 1555 1437 KP-150 150 5.164 37.70 35.44 268 254 1872 1388 NB-150 150 10.658 61.15 60.89 292 0 1430 533 NBP- 150 10.513 28.53 29.30 267 204 4680 1340 150 110 B-150 150 5.768 30.53 30.45 281 0 149 293

BP-150 150 5.477 78.20 30.42 262 206 9150 875 BMg- 150 5.497 0.1569 0.3820 242 189 1800 4075 150 T (C) pH U (wt%) U (wt%) Np in initial Mg in initial Np (ppm) Np (ppm) water acid solution solution water washed acid washed washed washed (ppm) (ppm) powder powder BMg- 150 5.497 0.1569 0.3820 242 185 1800 4075 150 T (C) pH U (wt%) U (wt%) Np in initial Ca in initial Np (ppm) Np (ppm) water acid solution solution water washed acid washed washed washed (ppm) (ppm) powder powder BCa-150 150 5.71 0.3277 0.3573 264 200 368 453

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CHAPTER 8:

THERMOGRAVIMETRIC ANALYSIS AND THERMAL POWDER X-RAY

DIFFRACTION OF SELECT URANYL PHASES

- The majority of uranyl minerals are hydrated, with H2O and (OH) groups playing a significant yet not fully understood structural role (Burns 2005). In particular, sheets of polyhedra of higher bond-valence make up the majority of uranyl minerals and the interlayer constituents of these structures are often overlooked in regards to their integral role in determining the stability of the minerals under geochemical conditions. Several studies have examined the role of interlayer species in uranyl minerals (Cejka et al 1999;

Finch et al 1998; Schindler et al. 2004a; Schindler et al. 2004b; Suzuki et al. 2005). For example, Schindler et al. (2004) reported differences in edge-site stabilities of minerals that share an anionic-sheet topology due to the differences in interstitial species arrangements. Water is a very common component of the interlayer complex of these structures, and hydroxyl groups are also commonly found as part of the structural units.

Water plays several different structural roles in uranyl minerals. Where water occurs in the interlayer of a structure it can be bonded to an interlayer cation. In this case,

H bonds emanate from the water group to extend to either the other interlayer water groups or to anions within the structural units (generally in the case of sheets). Water can also occur in the interlayer without direct bonding to a cation, in which case it is held in

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place by a network of H bonds. In this case, the O atom of the water group usually accepts two H bonds that are donated by other water groups in the interlayer, or by the structural unit. These water groups also donate two H bonds, either to interlayer water groups or to anions of the structural unit, thus the O atom of the water group is essentially held by H atoms in tetrahedral coordination (Hawthorne 1992; Burns 1999). Where hydroxyl groups occur within the structural unit, the H bond is usually donated to a water group contained in the interlayer, thus these hydroxyl groups relate directly to the role of water in the structure (Burns 1999).

With exposure to a dry environment under ambient conditions or with additional heating, uranyl minerals begin to dehydrate. Schoepite and autunite in particular have been shown to be very sensitive to dryness (Finch et al. 1998, Locock and Burns 2003).

Repeated wetting and drying cycles that may occur regularly in geochemical environments where uranyl minerals form, or in a geologic repository for nuclear waste such as Yucca Mountain, can be expected to greatly impact the stabilities and detailed compositions of such minerals. However, the role of the different types of water groups in determining the structural stability of uranyl minerals is poorly understood, and requires further study. For example, water groups held within a structure by H bonds alone can be expected to behave differently than water that is bonded to an interlayer cation or structural unit (Cejka et al 1999). Some water groups may be central to the stability of the structure, whereas others can be lost without destroying the structural connectivity and resulting in structural collapse. There have been several thermogravimetric analyses of uranyl minerals (e.g., Cejka et al. (1999), Gevorkyan et al.

(1979), and Urbanec et al. (1985)).

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8.1 Experimental Procedures

Synthetic powders of uranophane, becquerelite, soddyite, kasolite, and Na compreignacite were synthesized as described in Chapter 4. Each phase for this experiment underwent thermogravimetric analysis to determine a thermogravimetric analysis curve unique to the phase. Thermogravimetric analysis data were collected for each phase. Data were collected while heating at a consistent rate of 10˚C/min up to

900˚C for uranophane, Na compreignacite, soddyite and kasolite and up to 700˚C for becquerelite. The thermogravimetric analysis process tracks the loss of weight with temperature, and water is expected to account for most of the weight loss in these materials. Thermogravimetric analysis curves for all synthetic powders studied in this

Chapter are shown in Figure 8.1.

Once the thermogravimetric analysis was complete, a small amount of powder from the same synthesis of each phase, approximately 3 mg, was ground to a fine powder with the addition of an internal standard (Si), and placed upon a platinum heat stage filament contained in an Edmond Buhler HDK 1.4 apparatus attached to a Scintag X-ray diffractometer. Data were collected over the two-theta range of 10-80 degrees with a step size of 0.02 and a preset holding time of 2.5s in order to collect detailed patterns with good peak to background ratios.

The powder and Si mixture were prepared directly on the Pt thermal filament of the heat stage apparatus. A thermocouple is directly attached to the back of the filament to give the temperature of the filament throughout the heating process. In each case, an initial diffraction pattern was collected at 30˚C, and another was collected at 50˚C.

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Subsequently, diffraction patterns were collected in 25˚C increments, and the heating rate between data collections was 10˚C/min. In the case of kasolite, a pattern was collected every 50˚C up to 800˚C, and then every 25˚C from 800˚C to 1000˚C. The thermogravimetric instrument has a maximum heating temperature of 900˚C and the heat stage apparatus attached to the Scintag powder X-ray diffractometer is only consistently stable up to 1,000˚C, therefore no phase was analyzed higher than 1,000˚C.

The powder X-ray diffraction process requires holding the elevated temperature for two hours due to the data collection conditions. In order to verify that holding a sample at a given temperature did not have an effect on the mass loss of the phase, thermogravimetric analysis were conducted on the more hydrous uranyl phases analyzed in this experiment and held at various intervals to determine the stability when held at temperature (Figure 8.2 and 8.3). To verify this in full, powder X-ray diffraction patterns were collected under the same conditions used in this experiment for eight repetitions at

350˚C for synthetic powders of uranophane, which is shown in Figure 8.4.

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115

Figure 8.1 Thermogravimetric analysis curves for synthetic powders of kasolite, Na compreignacite, uranophane, soddyite, and becquerelite.

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116

117

118

Figure 8.4 Powder X-ray diffraction patterns taken for uranophane at 350˚C, while the specimen was held at temperature over the course of 12 hours.

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8.2 Unit Cell calculations

In order to track unit-cell change with temperature, powder patterns were collected for powders over the two-theta range of 10-80 degrees with a step size of 0.02° and a hold time of 2.5s. This procedure gives detailed patterns with even backgrounds.

Each pattern was analyzed for peak positions and corresponding two-theta values using

JADE version 8. The two-theta values were then corrected using an internal standard.

The corrected two-theta values were used with the cell refinement program CELREF to refine unit-cell dimensions. Starting cell dimensions and angles were taken from the room temperature values. Reflections were indexed where no ambiguity existed.

The program reports the number of two-theta reflections that correspond to the unit cell and calculates the resulting cell parameters following least-squares refinement.

Care was taken to examine the powder X-ray diffraction patterns before refining unit-cell parameters. Overlapping peaks are common and only powder X-ray diffraction patterns that match the corresponding International Crystallography Diffraction Database (ICDD) patterns of the possible unit cell were calculated.

CELREF is a Fortran program written by Daniel E. Appleman (1973), modified by Dr. L. Trembath and P.C. Burns. The program can index the 2θ values collected for each diffraction pattern followed by least-squares refinement of the unit cell dimensions for the phase of interest.

8.2.1 Internal Standard

Silicon was used as an internal standard as it has a consistent and well known thermal expansion that is minor over the thermal range explored in this project. By using

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Si as an internal standard, corrections were applied for variation in sample thickness and height on the thermal stage as the powder may experience expansion and contraction throughout the heating process. The powder X-ray diffraction patterns contain the peaks of Si. The platinum filament may also contribute peaks in the powder X-ray diffraction patterns, in particular at high temperature. A powder X-ray diffraction pattern of Si lightly coating the filament to allow the Pt peaks to be exposed is shown in Figure 8.5 with corresponding ICDD overlay patterns and identification numbers.

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8.3. Results for becquerelite

8.3.1 Thermogravimetric analysis for synthetic becquerelite

Becquerelite, Ca[(UO2)3O2(OH)3]2(H2O)8, is a sheet-type structure composed of

+2 uranyl pentagonal bipyramids, with Ca and H2O groups in the interlayer as discussed in further detail in Chapter 3. Becquerelite contains 8 H2O groups that comprise ~7.30 wt.% of the mass of the structure and 6 OH groups that comprises 5.18 wt.% of the structure. The thermogravimetric data shown in Figure 8.6 indicates that between 30˚C and 75˚C there is a slight weight loss of 0.64%. There is a sharp decrease in weight over the range of 75 to 150˚C, totaling 5.20%. From 150˚C to 275˚C there is a weight loss of

1.11%, corresponding to a total mass loss of 6.95%, which correlates to nearly all the water groups present in the structure. Additional heating results in the loss of hydroxyl.

From 275˚C – 450˚C there is a weight loss of 2.71%. From 450˚C to 900˚C there is shallow loss of 0.86%, for a total mass loss of 10.53%.

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Figure 8.6 Thermogravimetric analysis curve for becquerelite from 30˚C to 700˚C.

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8.3.2 High temperature powder X-ray diffraction for becquerelite

Powder X-ray diffraction patterns were taken every 25˚C with an initial pattern taken at 30˚C (Figure 8.7, with an overlay of the ICDD pattern for becquerelite). The unit cell for becquerelite was refined from the data through 75˚C, unit-cell parameters are in

Table 8.1 and in Figure 8.8. All powder X-ray diffraction patterns taken from 30°C to

500°C are given in Figure 8.9. The TGA data shows a significant weight loss in the range 75-125°C (Figure 8.6), and the powder diffraction patterns clearly demonstrate changes of the structure by ~75°C. This is most apparent when considering the peak at

~11° 2θ, which is a major feature of the becquerelite pattern, and is absent in patterns collected above 75°C.

The diffraction patterns indicate a new phase is present over the range of 100˚C-

375˚C. This phase is most evident when considering the emergence of a peak at ~14° two-theta (Figure 8.10 and 8.11). This phase most likely corresponds to the collapse of the structure prior to re-crystallizing. By 375˚C there has been ~9% mass loss and the powder X-ray diffraction data matches the ICDD pattern for U3O8 (shown in Figure

8.12). From 375˚C-500˚C the U3O8 phase is the only identifiable phase present (Figure

8.13). By 450˚C there is a mass loss of 9.50% which corresponds to complete dehydration and dehydroxyation. The additional mass lost from 450˚C to 900˚C must be due to O2- loss. The powder X-ray diffraction patterns for the temperature range 500˚C to

800˚C are in Figure 8.14. Another phase emerges at 650˚C and persists through 800˚C.

The final product appears to be a mixture of UO2 and U3O8, (Figure 8.15). There is no identified phase that contains Ca and it may be present in an amorphous phase.

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Figure 8.7 Powder X-ray diffraction pattern for becquerelite at room temperature with an overlay of the peak positions for becquerelite.

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TABLE 8.1

CALCULATIONS FOR BECQUERELITE CELL

T ˚C a b c

30 13.818(1) 12.369(8) 14.95(1)

50 13.821(9) 12.374(7) 14.948(9)

75 13.830(8) 12.360(7) 14.930(9)

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Figure 8.8 Graphical representation for the a, b, and c unit-cell dimension for becquerelite versus temperature.

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Figure 8.9 Powder X-ray diffraction patterns with the corresponding temperature listed along the Z axis for synthetic becquerelite as the starting material.

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Figure 8.10 Powder X-ray diffraction patterns for synthetic becquerelite at 30˚C-100˚C at 25˚C intervals.

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Figure 8.11 Powder X-ray diffraction patterns for starting material of synthetic becquerelite at 100˚C-300˚C at 25˚C intervals.

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Figure 8.12 Powder X-ray diffraction patterns for powders identified as becquerelite prior to heating, at 275˚C with an overlay of the peak positions for U3O8.

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Figure 8.13 Powder X-ray diffraction patterns for powders identified as becquerelite at room temperature, over the temperature range of 300˚C to 500˚C at 25˚C intervals

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Figure 8.14 Powder X-ray diffraction patterns for powders of synthetic becquerelite identified at room temperature, at 500 - 800˚C, at 25˚C intervals.

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8.4 Results for uranophane

8.4.1 Thermogravimetric analysis for synthetic uranophane

Uranophane, Ca[(UO2)(SiO3OH)]2·5H2O, consists of uranyl ions coordinated by five ligands arranged at the equatorial vertices of pentagonal bipyramids. The structure details are described in Chapter 3. The interlayer region contains Ca cations and H2O groups that are either bonded to Ca or held in the interlayer by H bonding only. There are

5 H2O groups in the interlayer of uranophane which corresponds to ~10.56% of the mass of the structure and 2 OH- which corresponds to 3.99% of the mass.

The thermogravimetric analysis curve for uranophane in Figure 8.16 shows a steep drop in mass from 30˚C to 200˚C with a mass loss of 8.51%, which corresponds to about 4 water groups. The remaining mass loss occurs gradually from 200 to 600˚C, with a mass loss of 4.46%, followed by a plateau from 600 to 900˚C with almost no mass. By

900˚C there is a total mass loss of 13.39%, which corresponds to the mass of the H2O groups, leaving 2.83% additional mass lost, this correlates to nearly all of the structural

OH-.

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Figure 8.16 Thermogravimetric analysis curve for synthetic powders of uranophane from 30-700˚C

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8.4.2 High Temperature Powder X-ray diffraction for synthetic powders of uranophane

The powder X-ray diffraction pattern collected at 30˚C for uranophane, with an overlay of the corresponding ICDD pattern, is shown in Figure 8.17. The pattern includes peaks from the Si internal standard. All patterns collected from 30˚C to 500˚C at 25˚C intervals are given in Figure 8.18. Uranophane is present to 125˚C, although the intensity of the prominent peak at ~11° 2θ diminishes significantly above 100°C. Unit cell dimensions of uranophane over the temperature range 25 to 125°C are given in Figure

8.19 and Table 8.2.

By 150°C the major uranophane peak at ~11° 2θ is lost (Figure 8.20). The specimen continues to diffract well, indicating that uranophane has been replaced by another crystalline phase or phases. The TGA data show that by 150˚C ~3 of the 5 H2O water groups are lost. By 200°C there is a total mass loss of ~8.5%, corresponding to four of the water groups present in the structure.

The diffraction pattern at 275°C reveals a poorly crystalline material, as evidenced by broad peaks and a high background (Figure 8.21). Suitable matches to these patterns were not found in the ICDD database. It is possible that this material represents a uranophane-like material without interlayer water, a scenario that is supported by its behavior when placed in water (see below). Powder patterns taken in the range of 300 - 500˚C are shown in Figure 8.22. By 500˚C the TGA analysis indicates all water groups and hydroxyls are lost. Heating past 500˚C gives little change in the powder

X-ray diffraction patterns (Figures 8.23 and 8.24). The diffraction patterns of powders at

900°C are shown in Figure 8.25. Peaks for Si, Pt, UO2 and CaUO3 are shown from the

ICDD. Although some peaks match UO2, it is uncertain if U was reduced to tetravalent

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while heating in air. A mixture of U3O8 and UO2 would be more likely. The powder following the heat cycle was black and brittle, consistent with reduction of uranium below the hexavalent state.

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Figure 8.17 Powder X-ray diffraction patterns for of powders of synthetic uranophane at 30˚C with an overlay of the ICDD pattern corresponding to uranophane.

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Figure 8.18 Powder X-ray diffraction patterns for powders identified as synthetic uranophane at room temperature, from the range 30˚C to 500˚C at 25˚C intervals.

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TABLE 8.2

UNIT CELL DIMENSIONS FOR URANOPHANE

T ˚C a b c β

30 15.900(8) 7.000(3) 6.684(3) 97.35

50 15.893(8) 7.006(3) 6.681(3) 97.29

75 15.906(5) 7.013(3) 6.677(5) 97.33

100 15.744(9) 6.997(6) 6.654(7) 96.59

125 15.80(3) 6.96(2) 6.69(1) 96.22

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Figure 8.19 Graphical representation of the a, b, and c unit-cell dimensions of the uranophane cell versus temperature.

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Figure 8.20 Powder X-ray diffraction of powders initially identified as synthetic uranophane from 30˚C to 125˚C showing a clear match for uranophane until 125˚C where broadening and peak loss can be seen.

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Figure 8.21 Powder X-ray diffraction of powders identified as synthetic uranophane at room temperature, showing overlays for the temperature range from 125 - 300˚C at 25˚C intervals.

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Figure 8.22 Powder X-ray diffraction patterns for powders initially identified as synthetic uranophane for the temperature range of 300˚C to 500˚C.

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Figure 8.23 Powder X-ray diffraction patterns of powders initially composed of synthetic uranophane for the range of 500 - 675˚C at 25˚C intervals.

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Figure 8.24 Powder X-ray diffraction patterns for powders initially identified as synthetic uranophane from 700 - 900˚C at 25°C intervals.

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Figure 8.25 Powder X-ray diffraction pattern of powders initially identified as synthetic uranophane at 900˚C with an overlay of peak

positions for Si, Pt, UO2, and CaUO3.

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8.4.3 Uranophane rehydration

Uranophane can be rehydrated after removal of some structural water.

Uranophane rehydrates after heated to a maximum temperature of 275˚C. Based upon the thermogravimetric analysis and powder X-ray diffraction patterns uranophane appears to structurally collapses after 125˚C, resulting in a uranophane-like phase, perhaps without interlayer water. A powder X-ray diffraction pattern for uranophane rehydrated after heating to 300˚C does not indicate uranophane is present (Figure 8.26). When the powder is heated to 275˚C and cooled, “rehydration” gives uranophane (Figure 8.27).

Rehydration consists of cooling the sample and adding one drop of water and allowing the sample to dry. After this temperature full irreversible damage or destruction occurs.

Up to 275˚C minimal reordering of the sheet structure may occur, thereby allowing rehydration to occur, but there is no noticeable change in the powder X-ray diffraction patterns from 275 to 300˚C. This was a unique feature of the five uranyl phases studied in this project, but is seen in clays minerals such as palygorskite

(Mg,Al)2Si4O10(OH)·4(H2O), a 2:1 type phyllosilicate that has been shown to rehydrate after being heated up to 300˚C. Dehydration was irreversible once heated to 400˚C

(Kuang et al. 2004).

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Figure 8.26 Synthetic powders initially identified as uranophane heated to 300˚C, then cooled and rehydrated with the addition of a

drop of ultra pure H2O, showing the absence of uranophane.

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Figure 8.27 Synthetic powders initially identified as uranophane heated to 275˚C, then cooled and rehydrated with the addition of a

drop of ultra pure H2O, showing a reappearance of uranophane.

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8.5 Results for soddyite

8.5.1 Thermogravimetric analysis for synthetic powders of soddyite

Soddyite, (UO2)2(SiO4)(H2O)2, has a framework of silicate tetrahedra and uranyl pentagonal bipyramids. For soddyite, chains of silicate tetrahedra share an edge with chains of uranyl pentagonal bipyramids of an adjacent chain, resulting in a three- dimensional framework structure. The 2 H2O groups are located at unshared equatorial vertices of the uranyl pentagonal bipyramids, and H bonds provide additional linkages between the uranyl silicate chains. The H2O groups make up 5.39 wt.% of the total soddyite structure (further descriptions and images can be found in Chapter 3).

Structurally soddyite is very different from the other phases studied here. Sheet structures generally have water weakly bound in interlayer cavities, whereas soddyite is a framework with water molecules structurally bound to the uranyl pentagonal bipyramids.

This should retard dehydration.

The thermogravimetric analysis curve in Figure 8.28 shows little change from

30°C to 300°C with a mass loss of only 0.63%, followed by a drop in mass from 300˚C to

500˚C with a loss of mass of 5.50%. The low-temperature mass loss is probably not structural water, and the weight loss between 300 and 500C corresponds to all the water in the structure. There is an additional mass from 700 - 750˚C with a tapering slope through to 900˚C corresponding to 1.5%. This mass loss presumably is oxygen, and is associated with reduction of some of the U to the tetravalent or pentavalent oxidation state.

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Figure 8.28 Thermogravimetric analysis curve for powder of synthetic soddyite from 30 - 700˚C

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8.5.2 High Temperature Powder X-ray diffraction for soddyite

A powder X-ray pattern for soddyite at 30˚C is shown in Figure 8.29 with an overlay of the peaks for the soddyite pattern from the PDF. Examination of patterns collected at elevated temperatures revealed that soddyite persists to ~400C. The refined unit cell shows that the a dimension steadily increases from 30˚C to 200˚C, whereas the b dimension decreases steadily from 30˚C through 200˚C. Discontinuities in the cell dimensions occur at 200°C (Figure 8.30 and Table 8.3). By 400˚C the structure of soddyite has been destroyed.

Discrete sections of the high temperature powder X-ray diffraction patterns from

30C to 500C can be seen in Figures 8.31, 8.32, and 8.33. There does not appear to be any other well crystallized phases between 300 and 500˚C. The soddyite peaks have lost almost all intensity and become broad with possibly some amorphous material contributing to the background. There is no evidence of a uranyl oxide such as UO3 or

U3O8. This implies amorphization has occurred.

The temperature range from 500˚C to 900˚C shows increasing crystallization of the final products with the last peaks emerging around 800°C, following O2- loss (Figure

8.34). The final products are likely a mixture of U3O8 and amorphous silica. Powder diffraction file overlays showing the peak positions for U3O8 are in Figure 8.35.

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Figure 8.29 Powder X-ray diffraction pattern for synthetic soddyite at room temperature with an overlay of the peak positions in the ICDD pattern for soddyite

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Figure 8.30 Graphical representation of the a, b, and c unit-cell dimension for synthetic soddyite versus temperature.

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TABLE 8.3

UNIT CELL DIMENSIONS FOR SODDYITE.

T˚C a b c 30 8.279(2) 11.267(2) 18.647(6) 50 8.299(1) 11.265(1) 18.647(3) 75 8.322(2) 11.256(2) 18.649(5) 100 8.341(1) 11.228(1) 18.624(2) 125 8.367(1) 11.210(2) 18.631(3) 150 8.399(3) 11.192(3) 18.630(5) 175 8.441(2) 11.178(5) 18.628(4) 200 8.297(2) 11.26(2) 18.64(2) 225 8.30(4) 11.29(5) 18.63(3) 250 8.29(3) 11.31(4) 18.63(2) 275 8.40(1) 11.38(4) 18.60(1) 300 8.38(1) 11.44(3) 18.59(1) 325 8.715(3) 11.196(4) 18.520(5) 350 8.176(2) 11.188(1) 18.747(2) 375 7.998(1) 11.671(5) 18.671(4)

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Figure 8.31 X-ray diffraction patterns for powder initially identified as synthetic soddyite over the temperature range of 30 - 175˚C at 25˚C intervals.

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Figure 8.32 Powder X-ray diffraction patterns for powder initially identified as synthetic soddyite over the temperature range of 200 - 400˚C at 25˚C intervals.

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Figure 8.33 Powder X-ray diffraction patterns for powder initially identified as synthetic soddyite over the temperature range of 300 - 500˚C at 25˚C intervals.

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Figure 8.34 Powder X-ray diffraction patterns for powder initially identified as synthetic soddyite collected over the temperature range of 500˚C – 900˚C at 25˚C intervals.

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8.6 Results for kasolite

8.6.1 Thermogravimetric analysis for synthetic powders of kasolite

Kasolite, Pb[(UO2)(SiO4)](H2O), is unique amongst the uranophane group of minerals that all contain sheets of uranyl pentagonal bipyramids and silicate tetrahedra.

Kasolite differs in that the apical anions of the silicate tetrahedra bond directly to the interlayer Pb2+ cations rather than forming an acid silicate group. All Pb2+ cations within the structure are coordinated by two OUr atoms, two O atoms of silicate tetrahedra, two O equatorial ligands of uranyl pentagonal bipyramids and silicate tetrahedra, and one H2O group. Further description and images can be found in Chapter 3. Kasolite contains the least amount of H2O of all the phases analyzed, closely followed by soddyite with 2 H2O groups. The one H2O group present in kasolite corresponding to 3.1% of the mass.

The thermogravimetric analysis data for kasolite in Figure 8.36 shows a long gently sloped loss of mass of ~3.77% with a flattening out at 450˚C. There is a nearly steady state between 450˚C and 850˚C with only 0.26% mass lost. At 850˚C there is a sudden mass loss of 0.42%, increasing the total loss of mass to 4.45%, implying the loss

2- of O as well as H2O. This correlates closely to the findings of Cejka et al. (1986), who collected thermogravimetric data and differential thermal analysis for kasolite and found a continuous mass loss up to 900˚C corresponding to 1 H2O group, and hypothesized that this water loss may be overlapped with O2- release and possibly PbO release.

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Figure 8.36 Thermogravimetric analysis curve for synthetic kasolite from 30˚C - 900˚C.

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8.6.2 High Temperature Powder X-ray diffraction for synthetic kasolite

An initial X-ray diffraction pattern taken for kasolite at 30˚C is shown in Figure

8.37 with an overlay of the matching ICDD pattern for kasolite. Diffraction peaks characteristic of kasolite persists to 550˚C (Figure 8.38), past the point that the thermogravimetric analysis data indicates the H2O group within the structure would have been lost. From the unit cell calculations there is a distinct decrease in the b dimension up to 350˚C, shown in Table 8.4 and Figure 8.39. By ~375˚C the thermogravimetric data indicates the loss of the one H2O group present in the structure, which may result in partial collapse but the high temperature powder diffraction patterns indicate kasolite is still present. The powder X-ray diffraction patterns taken between 550 and 800˚C demonstrate the decrease in crystallinity and loss of many of the peaks that can be seen in

Figure 8.40. At 650˚C there is a clear loss of the prominent kasolite peaks.

From 650 to 800˚C the powder begins to recrystallize, with peaks corresponding to a lead silicate (Figure 8.40). At 800˚C new peaks emerge indicating the crystallization of a new phase. This also corresponds to the last loss of mass seen in the thermogravimetric data, indicating the release of O2-and perhaps PbO. The final phase seen at 800˚C and above can be identified as a combination of U3O8 and PbO0.33 as shown in Figure 8.41. The final products as determined by powder X-ray diffraction data confirm the hypothesized final products speculated by Urbane et al. (1985) and Cejka et al. (1986).

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Figure 8.38 Powder X-ray diffraction patterns for powder initially identified as synthetic kasolite over the range of 30-800˚C at 50˚C intervals, 800-1000˚C at 25˚C intervals.

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TABLE 8.4

KASOLITE UNIT CELL

T˚C a b c 30 6.713(5) 6.933(4) 13.239(6) 50 6.735(5) 6.931(5) 13.249(6) 100 6.698(3) 6.927(1) 13.253(3) 150 6.604(3) 6.910(1) 13.265(2) 200 6.76(1) 6.902(3) 13.218(9) 250 6.731(5) 6.886(7) 13.248(9) 300 6.832(9) 6.876(5) 13.223(8) 350 6.765(7) 6.872(5) 13.23(1) 400 6.73(1) 6.885(8) 13.29(1) 450 6.76(1) 6.919(7) 13.16(1) 500 6.73(1) 6.847(6) 13.29(1) 550 6.717(4) 6.903(1) 13.253(2)

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Figure 8.39 Unit cell dimension and graphical representation of the a, b, and c unit-cell dimensions for kasolite versus temperature.

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Figure 8.40 Powder X-ray diffraction patterns for powder initially identified as synthetic kasolite for the temperatures range of 550 - 800˚C.

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Figure 8.41 Powder X-ray diffraction patterns for powder initially identified as synthetic kasolite at 1,000˚C with overlays of peak

positions corresponding to Si, Pt, PbO, and U3O8

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8.7 Results for Na compreignacite

8.7.1 Thermogravimetric analysis for synthetic Na compreignacite

Na-compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7, has not been structurally characterized, but X-ray powder diffraction data indicates that it is isostructural with compreignacite. The structure of compreignacite is a member of the α–U3O8-type sheet group, consisting of edge and vertex sharing uranyl pentagonal bipyrimids, with monovalent cations and H2O groups in the interlayer. The sheets are connected by both bonds to the interlayer Na cations and through a complex network of H bonding. There

- are a total of 7 H2O groups that make up 6.43% of the mass of the compound and 6 OH groups that comprise 5.21% of the structural mass (further details and images can be found in Chapter 3).

The thermogravimetric analysis curve in Figure 8.42 shows a 5.5% mass loss from 30˚C to 100˚C, indicating the H2O groups are very weakly bonded to the structure.

At this point approximately 5 water groups have been lost. There is a steady plateau from

75˚C to 200˚C that is followed by weight loss from 200 to 300˚C of 0.85% and then another weight loss from 350˚C to 440˚C of 2.78%. This brings the total mass loss to

9.13% by 440˚C. At this point all the water groups and at least two OH- groups have been lost. From 440˚C to 900˚C there is a gradual slope for an additional loss of 1.3%, giving a total mass loss of 10.42% through the course of heating. This corresponds to all water groups and either 4 OH- groups or a combination of OH- and O2-.

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Figure 8.42 Thermogravimetric analysis curve for synthetic sodium compreignacite from 30 - 900˚C.

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8.7.2 High Temperature Powder X-ray diffraction pattern for Na compreignacite

A powder X-ray diffraction pattern of Na compreignacite at room temperature with an overlay of the ICDD peak positions for Na compreignacite is shown in Figure

8.43. Na compreignacite quickly becomes unstable with increasing temperature and is lost by ~50°C, with the last peaks corresponding to Na compreignacite gone above 75°C.

The powder X-ray diffraction patterns collected from 30 - 500°C are in Figure 8.44 and a new phase begins to emerge by 50˚C and continues to persist through 325˚C (Figure 8.45 and 8.46). This compound is unidentified. This phase may be a Na compreignacite-like phase that has lost the interlayer water molecules. A different phase emerges by 375˚C and continues to increase in abundance through 500˚C (Figure 8.47). The phase that dominates from 375˚C through 500˚C is identified as Na0.35UO2.95, which is pattern 49-

1393 in the ICDD. The phase is hexagonal with an a dimension of 3.948 Å and a c dimension of 4.16Å. This implies that partial reduction of U has occurred.

The 6 hydroxyls in the Na compreignacite structure begin to be lost by 350˚C when at least two hydroxyl groups have been liberated. The OH- groups continue to be lost as the sample is heated. The in-growth of U3O8 as a final decomposition product can be seen over the range of 500 to 800˚C shown in Figure 8.48 and 8.49. The final products are a mixture of U3O8 and Na0.35UO2.95 shown in Figure 8.50 with the X-ray diffraction pattern of Na compreignacite at 800˚C.

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Figure 8.43 Powder X-ray diffraction pattern for Na compreignacite at room temperature with an overlay of the peak positions from the ICDD pattern for Na compreignacite.

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Figure 8.44 Powder X-ray diffraction patterns of powder initially identified as synthetic Na compreignacite for the temperature range 30 - 500˚C.

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Figure 8.45 Powder X-ray diffraction patterns for powder initially identified as synthetic Na compreignacite over the temperature range of 30 - 225˚C at 25˚C intervals.

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Figure 8.46 Powder X-ray diffraction patterns for powder initially identified as synthetic Na compreignacite over the temperature range of 200 - 350˚C.

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Figure 8.47 Powder X-ray diffraction patterns for powder initially identified as synthetic Na compreignacite over the temperature range of 300 - 500˚C.

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Figure 8.48 Powder X-ray diffraction patterns for powder initially identified as synthetic Na compreignacite over the range of 500 to 800˚C at 25˚C intervals.

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Figure 8.49 Powder X-ray diffraction pattern of powder initially identified as synthetic Na compreignacite at 800˚C with an overlay of

peaks corresponding to U3O8 shown as blue lines.

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Figure 8.50 Powder X-ray diffraction patterns for powder initially identified as synthetic Na compreignacite at 800˚C with an overlay of

peaks for U3O8 shown in the turquoise lines and Na0.35UO2.95 shown as the black lines.

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8.8 Discussion

The role of water within a uranyl structure has a significant impact on its stability.

The uranyl oxyhydrates becquerelite and Na compreignacite appear to be the least stable of the phases analyzed relative to H2O loss. Both becquerelite and Na compreignacite collapsed between 75˚C and 100˚C. Becquerelite contained the most H2O of all the phases analyzed with 8 H2O groups, and the thermogravimetric analysis data and powder

X-ray diffraction patterns, indicates the collapse of the structure occurs with the loss of only one H2O group between 75˚C and 100˚C.

Na compreignacite has the second most water of the structures analyzed with 7

H2O groups, all of which are lost by 75˚C. This indicates the water is weakly bonded and easily removed resulting in structural collapse. With additional heat the hydroxyls within the now collapsed structural unit are liberated, leaving behind only U, Na, and O for a combination of final products consisting of U3O8 and Na0.35UO2.95.

Uranophane retains its water groups until between 175˚C and 200˚C, at which point all H2O groups are lost. By 100°C uranophane cannot be identified by powder X- ray diffraction, and the structure has collapsed forming a intermediate phase. However, if heated only up to 275˚C then cooled and exposed to moisture, uranophane will reform.

This indicates the interlayer collapses but the sheet structure remains intact up to 275˚C.

Past 275˚C full rehydration of uranophane is impossible.

Soddyite is second to last in quantity of H2O groups with only 2, and it also retains H2O to a high temperature, retaining its structure up to ~400˚C at which point all

H2O groups are lost. Contrary to the phases discussed previously, the thermogravimetric

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analysis curve for soddyite is one smooth even curve up to 600˚C, followed by one steep drop in mass on the TGA curve, related to the loss of a structural O2-.

Kasolite has the least H2O of all phases studied and retains its structure up to

550˚C. This indicates the H2O is very strongly bonded in kasolite, though not as strong as predicted by Cejka (1999) who indicated the full water group was lost gradually all the way to 900°C. The thermogravimetric analysis curve for kasolite is very similar to the thermogravimetric analysis curve for soddyite. Both have a smooth even curve of mass lost, with an additional dip at the end of the heat cycle, for kasolite at 900˚C and for soddyite at 600˚C.

The thermogravimetric analysis and powder X-ray diffraction of uranophane and kasolite, which are both members of the uranophane sheet group, give very different results. Structurally the two phases are very similar; both are sheet structures with the uranophane sheet type. However, uranophane has 5 times as much H2O in the interlayer as kasolite, and has a smaller cation in the interlayer, Ca2+ in comparison to Pb2+. The only other notable difference between the two structures is that uranophane contains an

OH- bonded to Si rather than O2- as seen in kasolite. These differences result in very different behaviors as the structures are heated. Overall, kasolite behaves more like soddyite than uranophane and this may imply the sheet topology is not a controlling factor for phase behavior under heating. Rather, the quantity and bonding environment

- of the H2O groups and presence of weakly bonded OH groups may determine when structural break down and recrystallization will occur within a given phase. This indicates the interlayer is a very important aspect of the structural stability and durability in

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variable conditions such as wetting and drying episodes or heating and cooling fluctuations.

Prior to this study the final decomposition phases were only hypothesized as

UO2.67 or cation uranates. For uranophane, Grevorkyan et al. (1979) predicted CaU2O7 or

UO2.67, and Urabanec et al. (1985) predicted PbUO4 and amorphous silica for kasolite.

With the thermal attachment to the powder X-ray diffractometer actual intermediate phases, ranges and final decomposition products can be determined. For uranophane, no

CaU2O7 could be identified with confidence, and neither could PbUO4 in the case of kasolite.

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CHAPTER 9:

CONCLUSIONS AND OUTLOOK

The synthesis and structure determination of Na substituted metaschoepite discussed in Chapter 5 allows for the unique opportunity to examine the effect of an interlayer cation on Np5+ incorporation into a uranyl structure. The degree to which Na substitution in the interlayer of metaschoepite may increase the stability of the structure in unknown, but this effect could be substantial. This study has demonstrated that crystals of Na-MS will incorporate Np5+. Burns et al. (2004)

showed no incorporation of Np5+ in synthetic metaschoepite (which has

electroneutral sheets), and the current findings support the hypothesis that Np5+ incorporation is more likely in uranyl oxide hydrates with charged species in the interlayer.

Chapters 6 and 7 focus on understanding the crystal-chemical differences of Np5+ and U6+ through a series of Np incorporation experiments into several uranyl phases under varying conditions. Chapter 6 explored the relationship between synthesis temperature and solution pH on Np5+ incorporation into soddyite, uranophane, Na compreignacite, becquerelite, and kasolite. Np5+ was shown to incorporate into soddyite, verifying Np can be incorporated into framework structures. Np5+ incorporation was also seen for becquerelite, kasolite, uranophane and possibly Na compreignacite. The results for soddyite

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demonstrated a relationship to the maximum soak temperature, whereas for becquerelite, kasolite, and uranophane there is no clear relationship to the maximum synthesis temperature. The lack of Np5+ incorporation dependence on temperature observed for uranophane, becquerelite and kasolite may relate to the actual crystallization temperature, or the crystallinity of the products. An increase in crystallinity with increasing temperature may decrease Np incorporation if defects in the micro-crystals play a role in Np5+ incorporation. The solids may precipitate at any temperature over the heating cycle, not necessarily at the maximum temperature. Incorporation of Np5+ into kasolite and uranophane showed a strong relationship to the synthesis pH, particularly in the case of kasolite, with increasing pH increasing the quantity of Np5+ associated with the powders.

Chapter 7 examined the impact of additional cations in the synthesis solutions on Np5+ incorporation into uranyl phases that contain the uranophane type sheet. While there has been speculation in the literature and modeling to determine what cations may incorporate into boltwoodite, experimentation has demonstrated some of the complications related to the presence of additional cations. There is significant complexity involved in the interactions between uranyl phases and Np and only through experimentation can the actual behavior be determined.

Exploring the interactions between uranyl complexes and the neptunyl ion contributes an increased understanding of the uranyl phases as well as the behavior of the neptunyl ion. Determining the charge balancing mechanism or

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combination of mechanisms that makes Np incorporation possible also imparts information on structural pliability. By exploring the uranophane type sheet family of structures, structural variations amongst uranyl complexes were minimized, highlighting the effect of Np incorporation, and how the smallest

- variation in structure, such as more H2O groups, or an OH rather than the typical

O2- attached to the silicate tetrahedra can impact Np incorporation.

Chapter 8 explored the thermal stability of select uranyl phases by combining TGA and high temperature powder X-ray diffraction. This research increases understanding of the importance of H2O in relation to the structural stability of uranyl phases. The TGA curves for each phase vary significantly, depending upon the structural coordination and quantity of water present and how that water is bound within the structure. Uranophane, becquerelite, and Na compreignacite all quickly lost their interlayer water groups and structurally collapse. Uranophane showed the unique ability of the minerals studied to rehydrate following partial collapse. Kasolite and soddyite showed similar TGA curves and the ability to retain water groups up to 400°C in the case of soddyite and 550°C for the case of kasolite. The most common final decomposition phase identified was U3O8, although in some cases further reduction of uranium occurred, resulting in a small amount of UO2 present as well.

The temperature at which a phase loses its structural integrity is important in relation to the mineral stability when undergoing periods of wetting and drying under varying ranges of temperatures occurs, such is expected in a repository setting such as Yucca Mountain. In particular, the relatively low temperatures at

188

which becquerelite and Na compreignacite show collapse without the ability to rehydrate indicates these phases are unlikely to form prior to the repository cooling below 70°C. They can still form and be stable if there is enough water, but will collapse when they dry out and are exposed to higher temperatures.

The uranyl phases discussed within this dissertation are expected to be dominant alteration products that will form on altered spent nuclear fuel.

Understanding the behavior of these phases and how they interact with other radionuclides of interest such as Np, as well as wetting and drying events under varying thermal conditions is critical to predicting their presence and role in a nuclear repository or storage canister.

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APPENDIX A

ADDITIONAL POWDER X-RAY DIFFRACTION PATTERNS AND PHASE

IDENTIFICATION

190

191

Figure A.1 PXRD pattern for the wrap that is necessary to cover each Np rich uranyl powder synthesized at UND.

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Figure A.2 Becquerelite synthesized with Np with the matching ICDD pattern for becquerelite shown in blue.

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Figure A.3 Becquerelite matching PXRD patterns synthesized with Np at 80˚C, 100˚C, 120˚C, and 150˚C.

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Figure A.4 PXRD patterns of becquerelite synthesized with Np at 80˚C, 100˚C, 120˚C, and 150˚C, showing correlation to the pattern with the overlay ICDD identification pattern for becquerelite shown in figure A.1.

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195

Figure A.5 PXRD patterns for kasolite synthesized at 80˚C, 100˚C, 120˚C, and 150˚C, with an overlay pattern for the ICDD kasolite pattern shown in maroon.

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Figure A.6 PXRD patterns for kasolite synthesized with Np at 80˚C, 100˚C, 120˚C, and 150˚C.

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Figure A.7 PXRD patterns for kasolite synthesize with Np at a pH value of 4, 5, 6, 7, 8, 9, and 10, respectively.

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198

Figure A.8 PXRD patterns for U and UP, both synthesized with Np at 150˚C, with an overlay of the matching ICDD pattern for uranophane.

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Figure A.9 PXRD patterns for K and KP, both synthesized with Np at 150˚C, shown with a matching overlay ICDD pattern for kasolite

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200

Figure A.10 PXRD patterns for NB and NBP, synthesized in the presence of Np at 150˚C, with an overlay ICDD pattern for boltwoodite, as there is not an actual Na boltwoodite diffraction pattern available.

200

201

Figure A.11 PXRD patterns for Bo, BoP, BoMg, and BoCa, synthesized with Np at 150˚C with an overlay of the ICDD matching pattern for boltwoodite.

201

202

Figure A.12 PXRD patterns for Bo, BoP, BoMg, and BoCa.

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Figure A.13 IR data for kasolite, showing water molecules at 1300 and 300.

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Figure A.14 Soddyite IR pattern showing water molecules at 300 and 1350.

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