Crystal Chemistry and Structure of Kimzeyite, Ca3zr2[Al2si]

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2018-08-21 Crystal Chemistry and Structure of kimzeyite, Ca3Zr2[Al2Si]Σ3O12, henritermierite, Ca3Mn2[(SiO4)2(O4H4)1]Σ3, (OH,F)-spessartine, Mn2+3Al2[(SiO4)2(O4H4,F4)1]Σ3, and hausmannite, Mn3O4

Cruickshank, Laura Ann

Cruickshank, L. A. (2018). Crystal Chemistry and Structure of kimzeyite, Ca3Zr2[Al2Si]Σ3O12, henritermierite, Ca3Mn2[(SiO4)2(O4H4)1]Σ3, (OH,F)-spessartine, Mn2+ 3Al2[(SiO4)2(O4H4,F4)1]Σ3, and hausmannite, Mn3O4 (Unpublished master's thesis).. University of Calgary, Calgary, AB. doi:10.11575/PRISM/32836 http://hdl.handle.net/1880/107656 master thesis

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Crystal Chemistry and Structure of kimzeyite, Ca3Zr2[Al2Si]Σ3O12, henritermierite,

2+ Ca3Mn2[(SiO4)2(O4H4)1]Σ3, (OH,F)-spessartine, Mn 3Al2[(SiO4)2(O4H4,F4)1]Σ3, and hausmannite,

Mn3O4

Laura Ann Cruickshank

A THESIS

SUBMITTED TO THE FACULTY OF GRADUATE STUDIES

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE

DEGREE OF MASTER OF SCIENCE

GRADUATE PROGRAM IN GEOLOGY AND GEOPHYSICS

CALGARY, ALBERTA

AUGUST, 2018

ABSTRACT

This study considers the crystal chemistry of some rare garnet-group minerals of the general

[8] [6] [4] formula X3 Y2 Z3O12 including kimzeyite, Ca3Zr2[Al2Si]Σ3O12, henritermierite,

2+ Ca3Mn2[(SiO4)2(O4H4)1]Σ3, and (OH,F)-spessartine, Mn 3Al2[(SiO4)2(O4H4,F4)1]Σ3. Most garnets have cubic symmetry, with space group Ia3d, but a few uncommon exceptions have been reported with tetragonal symmetry, and space group I41/acd. A sample of kimzeyite, from type- locality, Magnet Cove, Arkansas, USA, a sample of henritermierite, from Wessels Mine X,

Kalahari manganese field, Northern Cape Province, South Africa, and a sample of (OH,F)- bearing spessartine, from Tongbei, near Yunxiao, Fujian Province, China were studied using electron-probe microanalysis (EPMA), back-scattered electron imaging (BSE), single crystal X- ray diffraction (SCXRD) and synchrotron high-resolution powder X-ray diffraction (HRPXRD).

For kimzeyite, structural Rietveld refinements confirmed cubic space group Ia3d and achieved reduced χ2 and overall R(F2) values of 1.840 and 0.0647, respectively. The kimzeyite sample contains an intergrowth of two cubic phases that began as oscillatory growth zoning, with later fluid-enhanced dissolution and re-precipitation giving rise to patchy intergrowths. For henritermierite and (OH,F)-bearing spessartine, the SCXRD structure refinements confirmed

2 tetragonal space group I41/acd, and produced a goodness of fit on F of 1.209 and 1.232 for henritermierite and (OH,F)-spessartine, respectively. In henritermierite, the deviation of unit-cell parameters from cubic symmetry is significant (a = 12.4908(2) Å, c = 11.6446(2) Å, c/a =

0.9534). Tetragonal henritermierite has a vacant Z2 site that contains the substituent O4H4 tetrahedron. The H atom is bonded to an O3 atom (O3 – H3) = 0.73(2) Å. Because of O2 –

3+ 3+ Mn – O2 Jahn-Teller elongation of the Mn O6 octahedron, a weak hydrogen bond is formed to the under-bonded O2 atom. This causes the large deviation from cubic symmetry. In (OH,F)- iii spessartine, the Z2 site is fully occupied, but the Z1 site contains vacancies. The Z1 and Z2 sites occupied by Si atoms are surrounded by four O atoms, as seen in cubic garnets. When the Z site is vacant, a larger [(O2H2)(F2)] tetrahedron is formed, and is similar to the O4H4 tetrahedron in hydrogarnets. These results indicate a new possible end-member:

2+ Mn 3Al2[(SiO4)2(O2H2)0.5(F2)0.5]Σ3, which remains unknown. Finally, this study also examines a tetragonal spinel mineral, hausmannite, ideally Mn3O4. The sample comes from the henritermierite specimen previously described, and has tetragonal space group I41/amd. The

SCXRD structure refinement confirmed the tetragonal symmetry, and produced a goodness of fit

2 on F of 1.144 and R indices of R1 = 0.0277 and wR2 = 0.0559. Henritermierite and hausmannite spinel occur as an intergrowth, observed in a BSE image. Hausmannite is thought to be an original mineral from which henritermierite was formed by a reaction that includes quartz, SiO2, calcite, CaCO3, and H2O.

ACKNOWLEDGEMENTS

I would like to express sincere thanks to my supervisor, Dr. Sytle Antao, for her guidance, encouragement and support during my time at the University of Calgary, during both my undergraduate and graduate degrees. Being given the opportunity to be a member of her research group for the last three years, through my undergraduate research project and my graduate degree has contributed significantly to both my personal and professional development.

Her passion for mineralogy has continually inspired me to never stop learning.

I would like to thank Dr. Robert Marr for his assistance with electron-probe microanalysis throughout the course of my research, and Benjamin Gelfand, for his assistance with single crystal X-ray diffraction data collection.

Inayat Dhaliwal, Melissa Greig, Han Nguyen, Joseph Ma, and Jeffrey Salvador are thanked for their valuable advice, discussions and support throughout my time at the University of Calgary.

Finally, I am most grateful to my family for their unconditional love and encouragement, and would like to thank my parents for raising me to always believe that I could do whatever I wanted to do and be whoever I wanted to be.

TABLE OF CONTENTS

Abstract...... ii Acknowledgements...... iv Table of Contents...... v List of Tables...... vi List of Figures...... viii

CHAPTER 1 INTRODUCTION TO THE GARNET GROUP MINERALS...... 1 1.1 Organization of the Thesis...... 2

CHAPTER 2 CRYSTAL CHEMISTRY OF KIMZEYITE...... 4 2.1 Introduction to Kimzeyite...... 4 2.2 Sample Description and Experimental Techniques...... 8 2.2.1 Electron-probe Microanalysis (EPMA)...... 9 2.2.2 Synchrotron High-resolution Powder X-ray Diffraction (HRPXRD)...... 12 2.2.3 Rietveld Structure Refinement...... 12 2.3 Results...... 13 2.4 Discussion...... 22 2.5 Conclusions...... 25

CHAPTER 3 CRYSTAL CHEMISTRY OF HENRITERMIERITE AND (OH,F)- SPESSARTINE ...... 27 3.1 Introduction to Henritermierite and (OH,F)-bearing Spessartine...... 27 3.2 Sample Description...... 31 3.2.1 Electron-probe Microanalysis (EPMA)...... 31 3.2.2 Single Crystal X-ray Diffraction (SCXRD)...... 33 3.2.3 Synchrotron High-resolution Powder X-ray Diffraction (HRPXRD)...... 34 3.3 Results...... 39 3.3.1 Henritermierite...... 41 3.3.2 (OH,F)-bearing Spessartine...... 47 3.4 Conclusions...... 51

CHAPTER 4 CRYSTAL CHEMISTRY OF HAUSMANNITE...... 52 4.1 Introduction to Hausmannite...... 52 4.2 Sample Description and Experimental Techniques...... 54 4.2.1 Electron-probe Microanalysis (EPMA)...... 55 4.2.2 Single Crystal X-ray Diffraction (SCXRD)...... 55 4.3 Results...... 56 4.4 Discussion...... 63 4.5 Conclusions...... 66

REFERENCES...... 67

Appendix A...... 78

LIST OF TABLES

Table 2.1: EPMA data for two kimzeyite phases...... 15

Table 2.2: HRPXRD data and Rietveld refinement statistical indicators for kimzeyite...... 17

Table 2.3: Atom coordinates, isotropic displacement parameters, U x 100 (Å2), and s.o.f.s for kimzeyite...... 19

Table 2.4: Selected bond distances (Å), angles (°) and differences between phases for kimzeyite...... 20

Table 3.1a: Electron-probe microanalysis (EPMA) and atoms per formula unit (apfu) for henritermierite from Wessels X Mine, North Cape, South Africa (ROM #M54234)...... 36

Table 3.1b: Electron-probe microanalysis (EPMA) and atoms per formula unit (apfu) for (OH,F)- bearing spessartine from Fujian Province, China...... 37

Table 3.2: SCXRD data for (a) henritermierite and (b) (OH,F)-bearing spessartine...... 38

Table 3.3: HRPXRD data and Rietveld refinement statistical indicators...... 39

Table 3.4: Atom coordinates, s.o.f.s and equivalent isotropic displacement parameters for (a) henritermierite and (b) (OH,F)-bearing spessartine...... 40

Table 3.5: Anisotropic displacement parameters (Å2) for (a) henritermierite and (b) (OH,F)- bearing spessartine...... 42

Table 3.6: Selected bond distances (Å) and angles (°) for (a) henritermierite and (b) (OH,F)- bearing spessartine...... 45

Table 3.7: Bond valence sums (v.u.) for (a) henritermierite and (b) (OH,F)-bearing spessartine...... 46

Table 4.1: Electron-probe microanalysis (EPMA) and atoms per formula unit (apfu) of hausmannite from Wessels X mine, North Cape, South Africa (ROM #M54234)...... 58

Table 4.2: Crystal structure refinement for hausmannite...... 60

Table 4.3: Atom coordinates and equivalent isotropic displacement parameters (Å2) for hausmannite...... 61

Table 4.4: Anisotropic displacement parameters (Å2) for hausmannite...... 61

Table 4.5: Selected distances (Å) and angles (°) for hausmannite...... 61

vii

Table 4.6: Bond-valence sums (v.u.) for hausmannite...... 61

Table 4.7: Unit-cell parameters and bond distances for hausmannite (listed with increasing V)..62

viii

LIST OF FIGURES

Figure 2.1: Polyhedral representation of the cubic kimzeyite structure composed of Ca2+ (X) dodecahedra (turquoise), Zr4+ (Y) octahedra (yellow) and Si4+ (Z) tetrahedra (grey). The structure is projected down the c axis...... 6

Figure 2.2: BSE image for kimzeyite. The upper-left region shows fine-scale oscillatory or lamellar zoning, parallel to the crystal faces. The right region contains light patches of different composition than the darker patches. The crystal contains two unique phases. The zoning and variation in composition arise from two different mechanisms: the zoning is an initial growth feature and the patches arise from fluid-enhanced dissolution and re- precipitation of a different garnet composition...... 10

Figure 2.3: BSE image and corresponding X-ray elemental maps for Al and Zr atoms in kimzeyite. (a) BSE image showing light and dark regions that correspond to two unique phases. The X-ray maps for (b) Al and (c) Zr atoms show a heterogeneous distribution. Maps for Ti and Fe atoms were created but have been omitted because of a lack of visible heterogeneity...... 11

Figure 2.4: Distribution of the major cations in the (a) Y and (b) Z site from nine different points in the crystal, arranged with increasing Zr apfu. An inverse relationship occurs between the amount of Zr and Ti atoms in the Y site. The amount of Fe3+ is nearly constant in the Z site and varies in a narrow range for the Y site (0.15 to 0.22 apfu). An inverse relationship also exists between the Si and Al content in the Z site...... 16

Figure 2.5: a) Complete HRPXRD trace for kimzeyite. The difference curve (Iobs – Icalc) is shown at the bottom. Short vertical lines indicate allowed reflection positions. The intensities for the trace and difference curve that are above 30° 2θ are multiplied by 10. The inserts show two asymmetric peaks that arise from overlap of peaks from two cubic phases. (b) Expanded view for a narrow 2θ range showing that each of the four reflection peaks is clearly asymmetric and split into two because of two cubic phases. A cubic single-phase garnet has sharp, narrow and symmetrical peaks (not shown)...... 18

Figure 2.6: Structural variations for garnet samples from andradite to kimzeyite. The (a) average , (b) mean , (c) Y – O and (d) Z – O distances vary linearly with the a unit-cell parameter and they meet near a synthetic Ti-andradite (green triangle; Weber et al. 1975). The intervals on the y-axis are the same in each plot (0.13 Å). Literature data are from Peterson et al. (1995), Armbruster et al. (1998) and Chakhmouradian and McCammon (2005). Error bars from this study are smaller than the symbols. Kimzeyite has a larger a unit-cell parameter, Y – O and Z – O distance than any natural silicate garnet...... 21

Figure 3.1: Tetragonal henritermierite structure showing the linkages of the Y octahedra (blue), Z1 (light grey), and Z2 (dark grey) tetrahedra. The H atoms (orange) are bonded to O3 atoms (pink) when the Z2 site is vacant. The O1 (yellow) and O2 (purple) atoms are also ix

shown. The X1 and X2 dodecahedral sites are omitted for clarity. The structure is projected down [100] and the O – H bonds are clearly observed...... 28

Figure 3.2: Tetragonal (OH,F)-spessartine structure showing the linkages of the Y octahedra (green), Z1 (light grey), and Z2 (dark grey) tetrahedra. The O1 (yellow) and O2 (purple), O3 (pink), and F (teal) atoms are also shown. The X1 and X2 dodecahedral sites are omitted for clarity. The structure is projected down [001]; open channels parallel to the c axis occur when the Z2 site is vacant...... 30

Figure 3.3: (a) Plane-polarized light (PPL) and (b, c) cross-polarized light (XPL) images of a thick section of henritermierite. The XPL images show birefringence, but the PPL, XPL and BSE images (Figure 5) do not show any significant inhomogeneous features...... 32

Figure 3.4: (a) Plane-polarized light (PPL) and (b, c) cross-polarized light (XPL) images from a 80 µm-thick thin section of (OH,F)-bearing spessartine. Image c has been rotated to the same orientation as images a, b. The XPL images show birefringence and some lamellar features, but the PPL, and BSE images (not shown) do not show any significant inhomogeneous features...... 32

Figure 3.5: BSE image showing an intergrowth of henritermierite garnet (dark grey) and hausmannite (white)...... 44

Figure 3.6: Complete HRPXRD pattern for tetragonal (OH,F)-spessartine. (a) 0 – 50 ° 2θ range. The difference curve (Iobs – Icalc) is shown at the bottom. Short vertical lines indicate allowed reflection positions. Expanded traces of the (b) 200 and (c) 004 and 400 peaks are also given. The presence of the 200 reflection, along with the splitting of the 004 and 400 reflections indicates tetragonal symmetry...... 48

Figure 3.7: Comparison of experimental traces for (a) cubic (Ia3d) spessartine from Colorado (Smyth et al. 1990), (b) tetragonal (I41/acd) (OH,F)-spessartine and (c) tetragonal (I41/acd) henritermierite. The 200 reflection is absent in (a) but is observed in the tetragonal garnets (b, c)...... 49

Figure 4.1: Ball and stick representation of the tetragonal hausmannite structure composed of Mn2+ (turquoise), Mn3+ (black) and oxygen (pink) atoms. The structure is projected down the c axis. The black dashed box represents the unit-cell outline...... 53

Figure 4.2: BSE image for hausmannite. The bright white regions are the euhedral hausmannite crystals, whereas the darker grey regions are henritermierite...... 59

Figure 4.3: Structural variations for hausmannite. Average distance varies linearly with (a) the a unit-cell parameter, (b) the c unit-cell parameter, and (c) the cell volume (Å3). The black linear trend line is related to data from Bosi et al. (2002)...... 65

CHAPTER 1

INTRODUCTION TO THE GARNET GROUP MINERALS

This study is focused on several unique members of the garnet supergroup. Most garnet

[8] [6] [4] [4] minerals with the general formula, X3 Y2 Z3 O12, exhibit cubic symmetry with space group Ia3d, but a few uncommon exceptions have been reported with tetragonal symmetry and space group I41/acd. Garnet minerals are of particular interest because they are important rock-forming minerals in the Earth’s crust and upper mantle (Liebau et al. 1982). The supergroup contains an isomorphous series of six common end members, with true end-member composition being particularly rare (Deer et al. 1982). Of the end-members, the garnets are further categorized into two series: the pyralspites and the ugrandites. The pyralspites are comprised of pyrope,

2+ Mg3Al2Si3O12 , almandine, Fe 3Al2Si3O12 , and spessartine, Mn3Al2Si3O12 , whereas the ugrandites are comprised of uvarovite, Ca3Cr2Si3O12 , grossular, Ca3Al2Si3O12 and andradite,

3+ 4+ Ca3(Fe ,Ti )2Si3O12. Within the garnet supergroup, the hydrogarnet group also exists and forms a solid solution between grossular, Ca3Al2(SiO4)3 , and katoite, Ca3Al2(O4H4)3. The hydrogarnet group is comprised of cubic hibschite, Ca3Al2[(SiO4)2(O4H4)1]Σ3 , and

3+ hydroandradite, Ca3Fe 2[(SiO4)2(O4H4)1]Σ3 , as well as tetragonal henritermierite,

3+ Ca3Mn2[(SiO4)2(O4H4)1]Σ3 , and holtstamite, Ca3(Al,Mn )2[(SiO4)2(O4H4)1]Σ3 (Sacerdoti and Passaglia 1985). Garnets are a unique mineral group because of their chemical diversity, as well as their physical and structural properties (Novak and Gibbs 1971). Garnets are found over a variety of pressure and temperature conditions and exhibit variable chemical composition because of the substituent cations at specific atom sites. In the garnet general formula, the X and Y sites host divalent and trivalent cations, respectively, and the Z site typically hosts Si atoms. The three cation sites are: the 8-fold coordinated dodecahedral (X) site, the 6-fold coordinated octahedral (Y) site and the 4-fold coordinated tetrahedral (Z) site, comprising a unit-cell that contains 8 of 2

these molecules (Deer et al. 1982). The garnet structure is highly flexible, with 53 possible elements incorporating into the crystal structure of natural garnet (Allmann and Hinek 2007). The dodecahedral X cation site may be occupied by a variety of divalent cations, such as Ca, Fe2+, Mg or Mn2+. The octahedral Y cation site may be occupied by trivalent or tetravalent cations, such as Al, Cr3+, Mn3+, Fe3+ or Ti4+. Finally, Si, Fe3+ or an OH- group occupies the tetrahedral Z site, which forms characteristic ZO4 tetrahedra. All cubic minerals should be optically isotropic; meaning that light passing through the mineral only travels with one velocity, regardless of direction. In order to be truly isotropic, a mineral must not show any birefringence in cross-polarized light of the optical microscope (Allen and Buseck 1988). Garnet is typically isotropic, but several documented cases of birefringence exist, indicating anisotropy (Deer et al. 1982), including spessartine and members of the ugrandite series. While this anomalous anisotropy is well documented, it is not yet well understood. This anomalous optical anisotropy indicates that not all garnets have cubic symmetry. It is speculated that there is a relationship between chemical composition and birefringence (Allen and Buseck 1988) and hypotheses for the cause of optical anisotropy include: plastic deformation, rare-earth element substitution into the crystal structure, strain caused by lattice mismatch, cation ordering at the octahedral site, and incorporation of OH- groups (Allen and Buseck 1988).

1.1 Organization of the Thesis

This thesis is organized into three main sections. The first section discusses cubic kimzeyite garnet (Chapter 2), the second section compares and contrasts tetragonal henritermierite and (OH,F)-bearing spessartine garnets (Chapter 3), and the third section discusses hausmannite, a tetragonal spinel mineral (Chapter 4). Each section is written in the format of a scientific paper. Chapter 2 and 3 of this thesis has been published in Acta 3

Crystallographica (Antao and Cruickshank 2016; Antao and Cruickshank 2018). The results of this study have been presented at various academic conferences (Antao et al. 2018; Cruickshank and Antao 2017a; Cruickshank and Antao 2017b; Antao et al. 2017).

CHAPTER 2

CRYSTAL CHEMISTRY OF KIMZEYITE

2.1 Introduction to Kimzeyite

Kimzeyite has ideal end-member formula, Ca3Zr2[Al2Si]Σ3O12 , and is a rare zirconium- rich garnet. It has been found in three localities: the type locality Magnet Cove, USA,

Stromboli, Italy and Anguillara, Italy. Kimzeyite from the type locality Magnet Cove, USA occurs in a carbonatite rock from the Kimzey calcite quarry. The euhedral crystals are typically small (< 1 mm in diameter) and dark brown to black in color (Milton et al. 1961).

The sample from Stromboli, Italy is a shoshonitic basalt and kimzeyite was found with pyroxene, olivine and monticellite (Munno et al. 1980). Kimzeyite from Anguillara, Italy occurs as a rare accessory mineral in the “Pizzo Prato Pyroclastic Flow” with gehlenite,

2+ Ca2Al[AlSiO7] and hercynite, Fe Al2O4 (Schingaro et al. 2001).

[8] [6] [4] [4] The general garnet group formula is X3 Y2 Z3 O12, with 8 atoms per unit-cell and cubic space group Ia3d. The X site is dodecahedrally coordinated and hosts Mg, Ca, Mn2+ or

Fe2+ cations, the Y site is octahedrally coordinated and hosts Al, Cr3+, Fe3+, Ti4+, or Zr4+ cations, and the Z site is tetrahedrally coordinated and hosts Si, Al or Fe3+ cations, or OH- groups. The garnet structure is comprised of alternating ZO4 tetrahedra and YO6 octahedra with

X cations filling cavities to form XO8 dodecahedra, as shown in Figure 2.1, a polyhedral representation drawn using “Crystal Maker”. The O atom occupies a general position, while the X, Y, and Z cations occupy a special, fixed position. If a substitution on the octahedral Y site occurs, the Y – O bond distance will change linearly with respect to the size of the substituent cation. However, when a Y site substitution occurs, the Z – O and bond distance is only marginally affected (Ungaretti et al. 1995, Antao et al. 2015). 5

Our research group has published many papers on the structural properties of garnets

(e.g., Antao et al. 2013; Antao 2014a, b; Antao and Klincker 2014; Antao et al. 2015; Antao and Cruickshank 2018). This study on kimzeyite extends further our contribution to the characterization of garnet-group minerals (Antao and Cruickshank 2016). Zoning is common in minerals, especially in garnets and other minerals such as erythrite and apatite (e.g., Antao and Dhaliwal 2017, 2018).

Kimzeyite has a unique composition with respect to other garnet minerals with Al or

Fe3+ occupying two-thirds of the tetrahedral sites, rather than Si as is expected in the garnet structure. Additionally, almost all octahedral sites are occupied by tetravalent Zr4+ or Ti4+, rather than typical trivalent cations Al and Fe3+. Finally, kimzeyite has a large Si deficiency, containing less than 10% Si, compared to 30-40% Si in most garnets (Milton et al. 1961).

Kimzeyite from type locality, Magnet Cove, was first studied by Milton et al. (1961).

They reported a cubic unit-cell parameter, a = 12.46 Å and a chemical formula:

4+ 3+ 2+ 2+ 5+ 3+ 3+ 4+ Ca3.11(Zr 1.42Ti 0.40Mn 0.07Fe 0.07Nb 0.05)Σ2.01[Al 1.26Fe 0.98Si 0.94]Σ3.18O12. This chemical composition indicates chemical heterogeneity as Ti3+ atoms replace Zr4+ atoms in the octahedral site and Al and Fe3+ replace Si in the tetrahedral site (Antao and Cruickshank 2016).

Structural analyses on a single crystal of kimzeyite from Stromboli Italy, by Munno et al. (1980) reported a cubic unit-cell parameter, a = 12.365(2) Å and chemical formula:

4+ 4+ 3+ {Ca2.94Mg0.06}Σ3.00(Zr 1.21Ti 0.47Mg0.32)Σ2.00[Al1.00Fe 0.49Si1.51]Σ3.00O12. This formula shows that Mg2+ cations are distributed amongst dodecahedral and octahedral sites, Zr4+ and Ti4+ were located at the octahedral site and Al and Fe3+ atoms fulfilled the Si deficiency at the tetrahedral site (Munno et al. 1980).

Figure 2.1. Polyhedral representation of the cubic kimzeyite structure, composed of: Ca2+ (X) dodecahedra (turquoise), Zr4+ (Y) octahedra (yellow), and Si4+ (Z) tetrahedra (grey). The structure is projected down the c axis.

A single crystal of kimzeyite from Anguillara, Italy, was studied by Schingaro et al.

(2001). This study reported a cubic unit-cell parameter of a = 12.397(4) Å and formula:

4+ 4+ 3+ {Ca2.97Ba0.03}Σ3.00(Zr 1.12Ti 0.68Mg0.11)Σ1.91[Al0.81Fe 0.85Si1.33]Σ2.99O12, from a crystal that lacked compositional zoning.

A notable difference in composition between initial studies on kimzeyite from the type- locality Magnet Cove and the current study is observed in the titanium ion. As a result of initial analyses on kimzeyite, titanium is reported as Ti3+ (Milton et al. 1969) but in this study, and in analyses on the two Italian samples, titanium is reported as Ti4+ (Munno et al. 1980,

Schingaro et al. 2001, Antao and Cruickshank 2016). Studies completed on synthetic zirconium and titanium garnets confirmed that Zr4+ atoms could go into 8-fold dodecahedral and 6-fold octahedral coordination and Ti4+ atoms can go into 6-fold octahedral and 4-fold tetrahedral coordination in the garnet structure (Ito and Frondel 1967). The two Italian samples are similar in both chemistry and structure, but the sample from Magnet Cove exhibits significantly different chemical composition and unit-cell parameter. The Magnet Cove sample contains more Zr, Al and Fe atoms, but fewer Si atoms per formula unit (apfu), which gives rise to the differences observed in structural parameters (Antao and Cruickshank 2016).

Birefringence in garnet has been discussed for over a century (Brewster 1853, Mallard

1876) but its origin remains questionable. Many garnet species including: almandine, grossular, spessartine, andradite, uvarovite and henritermierite, have exhibited optical anisotropy under the polarizing microscope, which indicates that the samples are not optically cubic. Possible explanations for the birefringence have been given, including: strain induced by plastic deformation (Allen and Buseck 1988), large-scale twinning (Brown and Mason

1994), OH- group substitution (Allen and Buseck 1988) and cation order in the X and Y sites 8

(Allen and Buseck 1988). The latter is the most popular hypothesis and results in a symmetry reduction to lower than cubic. Previous studies have observed birefringence in garnets containing lamellar or oscillatory growth features, referred to as “chemical zoning” rather than the identification of two separate phases (Akizuki 1984, Jamtviet 1991, Ivanova et al. 1998,

Pollok et al. 2001). These birefringent garnets contain multiple cubic phases that likely grew epitaxically. Epitaxial growth occurs when one phase grows above another, in an ordered fashion. These unique phases are detectable using modern technology such as high-resolution powder X-ray diffraction (HRPXRD) (Antao et al. 2015), but may go undetected using other experimental techniques such as conventional powder X-ray diffraction (XRD) or single- crystal X-ray diffraction (SCXRD).

Kimzeyite has been reported with cubic symmetry, space group Ia3d (Milton et al.

1961, Munno et al. 1980, Schingaro et al. 2001, Antao and Cruickshank, 2016). This study examines a sample from the type-locality Magnet Cove, Arkansas, USA. The chemically zoned sample contains an intergrowth of two cubic phases, occurring as oscillatory growth zoning and secondary patchy intergrowths. The sample has a significantly different composition and unit-cell parameter than either of the previously studied Italian samples

(Munno et al. 1980, Schingaro et al. 2001).

2.2 Sample Description and Experimental Techniques

This study examines euhedral kimzeyite crystals from type-locality Magnet Cove,

Arkansas, USA, originating from the Kimzey calcite quarry. The crystals used were black in color and approximately 1 mm in diameter.

2.2.1 Electron-probe Microanalysis (EPMA)

Quantitative chemical compositions and backscattered electron (BSE) images were collected with a JEOL JXA-8200 WD-ED electron-probe microanalyzer (EPMA). The JEOL operating program on a Solaris platform was used for ZAF correction and data reduction. The wavelength-dispersive (WD) operating conditions were 15 kV accelerating voltage, 20 nA beam current and 5 µm beam diameter. The sample was loaded into a disc and secured with epoxy. Following the setting of the epoxy, the sample was polished on one side using a selection of progressively finer abrasives, beginning with coarse sand paper (P400) and ending with very fine diamond paste (1 µm), in order to achieve a surface free of scratches. The crystal shown in Figure 2.2 was flipped over and polished again, in order to create the X-ray elemental maps, seen in Figure 2.3. By flipping and polishing the other side of the crystal, the oscillatory zoned region seen in Figure 2.2 was lost and is not seen in Figure 2.3. X-ray elemental maps were created for four major elements: Zr, Al, Fe and Ti atoms. The standards used were: almandine-pyrope (Mg Kα), grossular (Ca Kα), almandine (Fe Kα, Al Kα, Si Kα), rutile (Ti Kα), spessartine (Mn Kα), chromite (Cr Kα) and zircon (Zr Kα). After analyses were completed, the chemical composition was calculated using the spreadsheet of Locock (2008).

200 µm

Figure 2.2. BSE image for kimzeyite. The upper-left region shows fine-scale oscillatory or lamellar zoning, parallel to the crystal faces. The right region contains light patches of different composition than the darker patches. The crystal contains two unique phases. The zoning and variation in composition arise from two different mechanisms: the zoning is an initial growth feature and the patches arise from fluid-enhanced dissolution and re-precipitation of a different garnet composition.

(a) (b) (c) BSE Al Zr

200 µm

Figure 2.3. BSE image and corresponding X-ray elemental maps for Al and Zr atoms in kimzeyite. (a) BSE image showing light and dark regions that correspond to two unique phases. The X-ray maps for (b) Al and (c) Zr atoms show a heterogeneous distribution. Maps for Ti and Fe atoms were created but have been omitted because of a lack of visible heterogeneity.

2.2.2 Synchrotron high-resolution powder X-ray diffraction (HRPXRD)

The kimzeyite sample was studied using synchrotron high-resolution powder X-ray diffraction (HRPXRD), performed at beamline 11-BM, Advanced Photon Source (APS),

Argonne National Laboratory (ANL). The crystal was crushed to a uniform fine powder using a corundum mortar and pestle. The crushed sample was loaded into a Kapton capillary (0.8 mm diameter) and rotated during the experiment at a rate of 90 rotations per second.

The data were collected at room temperature (296 K) to a maximum 2θ of approximately 50° with a step size of 0.001° and a step time of 0.1s per step. The HRPXRD traces were collected with a multi-analyzer detection assembly consisting of 12 independent silicon (111) crystal analyzers and LaCl3 scintillation detectors that reduce the angular range to be studied and allow rapid data acquisition. A silicon (NIST 640c) and alumina (NIST 676a) standard (ratio of 1/3 Si : 2/3 Al2O3 by weight) was used to calibrate the instrument and refine the monochromatic wavelength used in the experiment (λ = 0.41422(2) Å). Additional details of the experimental set-up are given elsewhere (Antao et al. 2008b; Lee et al. 2008; Wang et al.

2008). Similar experimental techniques were successfully used to examine other minerals

(Antao et al. 2002; Antao and Hassan 2002; Hassan et al. 2003; Hassan et al. 2004; Parise et al.

2005; Ehm et al. 2007; Antao et al. 2008a; Antao et al. 2009; Ehm et al. 2009; Skinner et al.

2011; Antao 2012).

2.2.3 Rietveld structure refinement

The Rietveld method was implemented in the GSAS program (Larson and Von Dreele

2001) with the EXPGUI interface (Toby 2001) to analyze the HRPXRD data. Scattering curves for neutral atoms were used. Starting atom coordinates, unit-cell parameter, and space group Ia3d, were taken from Schingaro et al. (2001). The background was modeled using a 13

shifted Chebyschev polynomial with eight terms. In the GSAS program, the reflection-peak profiles were fitted with a pseudo-Voigt (type-3) profile (Cagliotti et al. 1958, Thompson et al.

1987). A full-matrix least-squares refinement was carried out by varying the experimental parameters in the following order: scale factor, unit-cell parameter, atom coordinates and isotropic displacement parameters. Analysis of the HRPXRD trace shows that the sample is comprised of two unique phases. No impurities or un-indexed peaks were observed. The two unique phases were refined together using site occupancy factors (s.o.f.s) of the dominant atom in the X, Y, and Z cation sites. Towards the end of the refinement, all parameters were allowed to vary simultaneously and convergence was achieved.

2.3 Results

The kimzeyite sample is birefringent with inclined extinction to the crystal faces, under the optical microscope. The birefringence of kimzeyite is similar to that of Ti-rich andradites

(Antao et al. 2015) and indicates that kimzeyite is not optically cubic. BSE images revealed fine-scale oscillatory zoning (Figure 2.2) and patchy intergrowths with light and dark regions

(Figure 2.2 and Figure 2.3a). The occurrence of light and dark regions within the sample indicates variable composition. The lighter region corresponds to phase 1b, and the darker region corresponds to phase 1a. The different phases occur as light and dark regions because the lighter region is appearing brighter, due to a higher mean atomic number.

The X-ray elemental maps, shown in Figure 2.3, were obtained for four major elements:

Zr, Al, Fe and Ti atoms. Subtle variations in color contrast indicate a heterogeneous distribution of Zr and Al atoms, but significant variations were not seen in the Fe and Ti atoms, and these maps are not shown here. Quantitative chemical analyses from the nine studied points on the crystal indicate that the X cation site is filled with only Ca atoms, and cation 14

variation exists in the Y and Z sites, shown in Figure 2.4. Complete EPMA data are given for two points (point 1 and 9), corresponding to the two observed phases in Table 2.1.

Two unique compositions are suggested by BSE images, X-ray elemental maps and quantitative elemental analysis. The presence of two unique phases is easily confirmed with

HRPXRD data, observed as distinct diffraction features. 15

Table 2.1. EPMA data for two kimzeyite phases Oxide (wt.%) Phase 1a Phase 1b SiO2 12.73 11.51 TiO2 6.37 4.97 ZrO2 27.68 30.31 Al2O3 6.65 7.39 Cr2O3 0.03 0.00 FeO 15.20 13.57 MnO 0.09 0.04 MgO 0.00 0.00 CaO 28.94 27.95 Σ 97.69 95.74

Recalculated (wt. %) Final FeO 0.00 0.05 Final Fe2O3 16.89 15.02 Final MnO 0.01 0.04 Final Mn2O3 0.10 0.00 Σ 99.39 97.24

Cations for 12 O atoms (apfu) Ca2+ 3.000 2.992 Mn2+ 0.001 0.003 Fe2+ 0.000 0.004 ΣX 3.001 3.000 Zr4+ 1.306 1.477 Ti4+ 0.464 0.374 Fe3+ 0.220 0.150 Mn3+ 0.007 0.000 Cr3+ 0.002 0.000 ΣY 1.999 2.000 Si4+ 1.232 1.150 Fe3+ 1.010 0.980 Al3+ 0.758 0.870 ΣZ 3.000 3.000

End-member (mol %) Kimzeyite, † Ca3Zr2[Al2Si]Σ3O12 37.9 43.5 Kimzeyite-Fe, Ca3Zr2[Fe2Si]Σ3O12 27.4 30.3 Schorlomite, Ca3Ti2[Fe2Si]Σ3O12 23.1 18.7 Andradite, Ca3Fe2Si3O12 11.0 7.2 Quality Index Superior Excellent

Phase 1a = Ca3.00(Zr1.31Ti0.46Fe0.22Mn0.01)Σ2[Al0.76Fe1.01Si1.23]Σ3O12.

Phase 1b = Ca2.99(Zr1.48Ti0.37Fe0.15)Σ2[Al0.87Fe0.98Si1.15]Σ3O12. F(000) = 182 and 184 electrons for phases 1a and 1b, respectively. † Kimzeyite is the dominant end member. 16

0.4 (a) 0.46 1.45 ) ) 4+ 0.3 ) Zr 0.44 apfu 4+ apfu Ti apfu 1.40 3+ Fe 0.42 0.2 Y site (Fe Y site (Fe Y site (Ti Y site (Ti

Y site (Zr Y site (Zr 0.40 1.35 0.1

0.38 0.0 1 2 3 4 5 6 7 8 9 Points 1.6 0.86 1.22 1.4 ) 0.84 ) 1.2 ) 1.20 apfu apfu 1.0

apfu 4+ 1.18 Si 0.82 3+ 0.8 Al 3+ 0.80 0.6

1.16 Fe (Fe Z site Z site (Al (Al Z site Z site (Si Z site 0.4 1.14 0.78 0.2 (b) 1.12 0.76 0.0 1 2 3 4 5 6 7 8 9 Points

Figure 2.4. Distribution of the major cations in the (a) Y and (b) Z site from nine different points in the crystal, arranged with increasing Zr apfu. An inverse relationship occurs between the amount of Zr and Ti atoms in the Y site. The amount of Fe3+ is nearly constant in the Z site and varies in a narrow range for the Y site (0.15 to 0.22 apfu). An inverse relationship also exists between the Si and Al content in the Z site.

A complete HRPXRD trace for the kimzeyite sample is shown in Figure 2.5a. An expanded trace for a select 2θ range is shown in Figure 2.5b. The expanded trace shows that each reflection peak is asymmetric and exhibits peak splitting into doublets, caused by overlapping peaks from the two cubic phases. A HRPXRD trace of a single-phase cubic garnet would display sharp, symmetrical peaks with no broadening or splitting (Antao et al. 2015).

The unit-cell parameters and Rietveld refinement results are given in Table 2.2. Atom coordinates, isotropic displacement parameters and site occupancy factors (s.o.f.s) are given in

Table 2.3. Bond distances and angles are given in Table 2.4 and unit-cell parameters and select bond distances are presented graphically in Figure 2.6.

Table 2.2. HRPXRD data and Rietveld refinement statistical indicators for kimzeyite

Phase 1a Phase 1b wt. % 42.6 (2) 57.4 (2) a (Å) 12.46553 (3) 12.47691 (2) Δa (Å) = 1b – 1a - 0.01138 Reduced χ2 1.840 - R(F2) 0.0647 - †LY 9.8 8.8 Data points 47994 -

Nobs 1528 - λ (Å) 0.41422 (2) - † LY is related to strain and these values are large compared to those of a single-phase Ti-rich garnet with LY ≅ 3 (Antao, Zaman, Gontijo et al. 2015).

Figure 2.5. (a) Complete HRPXRD trace for kimzeyite. The difference curve (Iobs – Icalc) is shown at the bottom. Short vertical lines indicate allowed reflection positions. The intensities for the trace and difference curve that are above 30° 2θ are multiplied by 10. The inserts show two asymmetric peaks that arise from overlap of peaks from two cubic phases. (b) Expanded view for a narrow 2θ range showing that each of the four reflection peaks is clearly asymmetric and split into two because of two cubic phases. A cubic single-phase garnet has sharp, narrow and symmetrical peaks (not shown). 19

Table 2.3. Atom coordinates, isotropic displacement parameters, U x 100 (Å2) and s.o.f.s for kimzeyite.

Phase 1a Phase 1b 1a – 1b1 Ca (X) U 0.93 (2) 0.93 (2) - Zr (Y) U 0.622 (4) 0.622 (4) - Fe (Z) U 0.51 (2) 0.51 (2) - O x 0.0347 (1) 0.0343 (1) - y 0.0500 (1) 0.0498 (1) - z 0.6535 (2) 0.6538 (1) - U 1.65 (3) 1.65 (3) - s.o.f. 1.0 1.0 - Ca (X) s.o.f. 0.960 (4) 0.957 (3) 0.003 Zr (Y) s.o.f. 0.809 (3) 0.828 (2) -0.019 Fe (Z) s.o.f. 0.623 (2) 0.617 (2) 0.006 Ca (X) EPMA s.o.f. 0.994 0.994 - Zr (Y) EPMA s.o.f. 0.852 0.889 - Fe (Z) EPMA s.o.f. 0.661 0.669 - Ca (X) 1Δ (s.o.f.) -0.040 -0.044 - Zr (Y) Δ (s.o.f.) -0.046 -0.062 - Fe (Z) Δ (s.o.f.) -0.061 -0.061 - Ca (X) 1Δ e -0.8 -0.9 - Zr (Y) Δ e -1.8 -2.5 - Fe (Z) Δ e -1.6 -1.6 - 1 1a – 1b = difference between s.o.f.s obtained by refinements; a significant difference is seen in the Y site. Δ (s.o.f.) = s.o.f. (HRPXRD refinement) – s.o.f. (EPMA). Δe = electrons (HRPXRD refinement) – electrons (EPMA).

Table 2.4. Selected bond distances (Å), angles (°) and differences between phases for kimzeyite. Phase 1a Phase 1b 1a – 1b† Z – O x 4 1.761 (2) 1.762 (1) -0.001 Y – O x 6 2.059 (2) 2.062 (1) -0.004 X – O x 4 2.408 (2) 2.403 (1) 0.004 X’ – O x 4 2.555 (2) 2.560 (1) -0.005 [8] 2.4815 2.4818 -0.000 [4] 2.1957 2.1970 -0..001 Y – O – Z x 1 131.5 (1) 131.4 (1) 0.1

Radii Σ Z – O 1.750 1.753 - Y – O 2.064 2.073 - 2.500 2.499 - 2.204 2.206 - † 1a – 1b = difference in value between phases 1a and 1b. ( = {(Z – O) + (Y – O) +(X – O) + (X’ – O)}/4 ), and is the average distance from the four-coordinated O atom. 21

2.55 This study; Literature; Weber et al. (1975) Antao et al. (2015b) Schingaro et al. (2001); Munno et al. (1980)

2.50 /Å

2.45

(a)

12.1 12.2 12.3 12.4 2.25 a/Å

2.20 /Å

2.15 (b)

12.1 12.2 12.3 12.4 a/Å 2.10

/Å 2.05 Y-O

2.00 (c)

12.1 12.2 12.3 12.4 a/Å 1.76

1.74

1.72 /Å 1.70 Z-O 1.68

1.66 (d) 1.64 12.1 12.2 12.3 12.4 a/Å

Figure 2.6. Structural variations for garnet samples from andradite to kimzeyite. The (a) average , (b) mean , (c) Y – O and (d) Z – O distances vary linearly with the a unit-cell parameter and they meet near a synthetic Ti- andradite (green triangle; Weber et al. 1975). The intervals on the y-axis are the same in each plot (0.13 Å). Literature data are from Peterson et al. (1995), Armbruster et al. (1998) and Chakhmouradian and McCammon (2005). Error bars from this study are smaller than the symbols. Kimzeyite has a larger a unit- cell parameter, Y – O and Z – O distance than any natural silicate garnet. 22

2.4 Discussion

Quantitative EPMA results and HRPXRD data show that the kimzeyite sample from

Magnet Cove, USA contains phases 1a and 1b. From the EPMA data, two different compositions were found, corresponding to the two different phases found in HRPXRD data, but the difference between the two compositions is small. Data analysis resulted in the

4+ 4+ 3+ 3+ 3+ formulae: {Ca3.00}(Zr 1.31Ti 0.46Fe 0.22Mn 0.01)Σ2[Al0.76Fe 1.01Si1.23]Σ3O12 for phase 1a, and

4+ 4+ 3+ 3+ {Ca2.99}(Zr 1.48Ti 0.37Fe 0.15)Σ2[Al0.87Fe 0.98Si1.15]Σ3O12 for phase 1b. Phase 1b contains more Zr than phase 1a and the distribution of the Y and Z site cations is shown in Figure 2.4a and 2.4b. An inverse relationship is seen between Zr and Ti content in the Y site, and the amount of Fe3+ varies from 0.15 to 0.22 apfu. In the Z site, the amount of Fe3+ is consistent between the two phases, but the Si and Al content has an inverse relationship.

The Rietveld structure refinement confirms that phase 1b contains more Zr atoms than phase 1a. The Fe3+ and Al3+ cations replace smaller Si4+ cations in the tetrahedral Z site. This size difference does not result in a large difference in the site occupancy factors (s.o.f.s) for the

Z sites between the two phases, so both phases have almost the same Z – O distance, reported in Table 2.4. The Y site contains Zr4+, Ti4+ and Fe3+ cations, and the difference in s.o.f.s between the two phases is the largest in the Y site, causing 1b to have the largest unit-cell parameter and Y – O bond distance, because it contains the most Zr4+ atoms. The average

O> bond distances are the same in phase 1a and 1b, because both phases contain only Ca atoms in the X site. Kimzeyite has the largest bond distance of all natural silicate garnets.

Literature data from various single-crystal structure refinements are represented graphically in Figure 2.6. All of these studies collected data in the cubic space group, Ia3d, and only single phases were observed. In this figure, the red trend line to kimzeyite was fitted 23

with data from Weber et al. (1975) and data from this study. Weber et al. (1975) reported the crystal structure for a synthetic sample with composition:

4+ 3+ 3+ Ca3(Ti 1.42Fe 0.58)Σ2[Fe 1.42Si1.58]Σ3O12, where a change is seen in the trend line. Kimzeyite single-crystal data from Schingaro et al. (2001) and Munno et al. (1980) differ the most with respect to the Y – O and Z – O distances, but their average and distances are close to the trend lines of this study.

The black trend lines in Figure 2.6 are fitted to data points from Antao et al. (2015). All of the samples included here have unit-cell edges between 12.05 Å and 12.48 Å, corresponding to andradite and kimzeyite, respectively. Between the lowest (12.05 Å) and highest (12.48 Å) unit-cell edges are values for Ti-rich andradite and schorlomite. The bond distances obtained by the single crystal X-ray diffraction (SCXRD) and HRPXRD methods for kimzeyite are similar, but the Magnet Cove sample of this study has the largest unit-cell parameter reported for any natural garnet. None of the single-crystal studies observed multiple phases, because this experimental technique is not appropriate for obtaining structural parameters from a multi- phase sample. In order to resolve structural parameters of a multi-phase sample, it is necessary to use a technique with good resolution (such as HRPXRD), because the differences in structural parameters such as the unit-cell parameter are very small.

The oxygen atom is found on a general position in the garnet structure, whereas the cation sites all occupy fixed positions. As seen in Figure 2.1, each oxygen atom is coordinated by four cations: one Z, one Y and two X cations in tetrahedral configuration. Substitution at any of the four cation sites impacts each of the other cation sites, as seen in their bond distances. In kimzeyite, substitutions occur at both the Y and Z sites, but not at the X site, as it is solely occupied by Ca atoms. The Z – O distance in kimzeyite (1.76 Å) is much larger than 24

the Z – O distance (1.64 Å) when the Z site contains only Si, such as in andradite, ideally

3+ Ca3Fe 2Si3O12. The X site is filled with only Ca atoms, so the average distance (2.48

Å) in kimzeyite is significantly larger than the average of grossular, ideally

Ca3Al2Si3O12, (2.41 Å), in response to substitutions on the Y and Z sites (Antao 2015).

Therefore, the X site has only a minor influence on the structural variations in kimzeyite, but the atoms in the Y and Z sites dictate the observed variation.

3+ From anhydrous andradite, ideally Ca3Fe 2Si3O12, to end-member kimzeyite, ideally

Ca3Zr2[Al2Si]Σ3O12 , the Y – O distance is expected to increase because of the size of the substituent cation, Zr4+ (0.72 Å) as compared to that of Fe3+ (0.645 Å). Inversely, the substitution of smaller Ti4+ (0.605 Å) for Fe3+ (0.645 Å) causes a slight decrease in Y – O distance in Ti-rich andradites (Antao et al. 2015).

After the data point of Weber et al. (1975) in Figure 2.6, the Y – O distance increases significantly because of the substitution of Zr4+ for Fe3+ at the Y site. In Ti-rich andradites, the

Z – O distance increases because of the substitution of Fe3+ (0.49 Å) for Si4+ (0.26 Å) at the Z site, whereas substitution of Al3+ (0.39 Å) has minimal effect. In kimzeyite however, Al3+ substitution plays an important role, so the Z – O distance follows a separate trend line. The Ca atom in the X sites of Ti-rich andradites and kimzeyite are similar, so their average trend lines are also similar. Additionally, the Y – O, Z – O and average distances are nearly identical in phase 1a and phase 1b of the kimzeyite sample, as shown in Figure 2.6.

The distances ( = {(Z – O) + (Y – O) + (X – O) + (X’ – O)}/4) vary linearly with the a unit-cell parameter in cubic silicate garnets (Antao and Klincker 2013). The linear dashed line for was fit to data from Antao et al. (2015) and this study, shown in

Figure 2.6b. All of the data points shown in Figure 2.6 fall on the straight trend line for

O>, but the other distances fall on two lines and exhibit some scatter. This indicates that satisfactory coordination of the O atom is critical to the garnet structure.

The two cubic phases in kimzeyite occur as chemically zoned regions parallel to crystal faces and constitute fine-scale epitaxial or oscillatory growth. The patchy intergrowths of the two phases arise from a secondary process that involves fluid-enhanced dissolution and re- precipitation. These unique features are formed by two different mechanisms that are not yet fully understood. The zoning in kimzeyite involves major elements, instead of minor elements, which has been previously observed in some garnets. Zoning involving minor elements was recently observed in almandine garnet (Ague and Axler 2016). Changes in composition may occur as the crystal grows at relatively low temperature (less than 673 K), preventing diffusion or homogenization.

Patchy intergrowths observed in kimzeyite may form by a fluid process involving dissolution and re-precipitation (Putnis 2009, Ague and Axler 2016). High temperatures (1273 to 1673 K) are used to synthesize single-phase silicate garnets (Ganguly et al. 1993). When a crystal forms at high temperature, it may exsolve a second phase upon cooling, but the orientations of these two phases are related to each other. Although exsolution is expected in garnets (Cressey 1978, Yardley et al. 1996, Wang et al. 2000), it has not been observed yet to date. Strain arises from structural mismatch of the two cubic kimzeyite phases and gives rise to optical anisotropy.

2.5 Conclusion

Kimzeyite has cubic symmetry with space group Ia3d. The sample exists as an intergrowth of two cubic phases that occur as fine-scale oscillatory zoning parallel to the crystal faces and as patchy intergrowths. These features arise from two different mechanisms: 26

epitaxial growth and fluid-enhanced dissolution and re-precipitation, respectively. These features may be destroyed at temperatures ( > 1073 K) high enough for homogenization of the cations, resulting in a single cubic phase. The two-phase kimzeyite is anisotropic because of strain arising from structural mismatch, seen as differences in the unit-cell parameter and bond distances. The two cubic phases have a subtle compositional difference in the Y and Z sites.

Kimzeyite from Magnet Cove, USA has the largest unit-cell parameter, and Y – O and Z – O bond distances of any natural silicate garnet.

CHAPTER 3

CRYSTAL CHEMISTRY OF HENRITERMIERITE AND (OH,F)-BEARING SPESSARTINE

3.1 Introduction

3+ Henritermierite, ideally Ca3Mn 2[(SiO4)2(O4H4)1]Σ3 is a member of the hydrogarnet group of garnet minerals. Henritermierite is a rare Mn3+ bearing silicate garnet, found only in two manganese mines: one in Morocco and the other in South Africa. This mineral has tetragonal symmetry with space group I41/acd and is one of three hydrogarnet minerals with tetragonal symmetry, the other two being (OH,F)-bearing spessartine,

2+ Mn 3Al2[(SiO4)2(O4H4,F4)1]Σ3 (Boiocchi et al. 2012), and holtstamite,

3+ Ca3(Al,Mn )2[(SiO4)2(O4H4)1]Σ3 (Halenius et al. 2005).

Other hydrogarnet minerals have been reported with cubic symmetry, including: hibschite (hydrogrossular), Ca3Al2[(SiO4)2(O4H4)1]Σ3, and hydroandradite,

3+ Ca3Fe 2[(SiO4)2(O4H4)1]Σ3 (Armbruster et al. 2001) , both with space group Ia3d.

Henritermierite differs from the other hydrogarnets, as it displays ordered O4H4 units, whereas other hydrogarnets display disordered small SiO4 and larger O4H4 tetrahedra

(Armbruster et al. 2001). It can be differentiated from holtstamite and (OH,F)-bearing spessartine by the dominant octahedral Y site cation: Al in holtstamite and (OH,F)-bearing spessartine, and Mn3+ in henritermierite. The occurrence of Mn3+ in the octahedral Y site is observed in natural tetragonal hydrogarnets, but is severely limited in natural cubic garnets

(Halenius 2005).

In the crystal structure, Ca and Mn3+ ideally occupy the dodecahedral X site and octahedral Y site, respectively, and Si occupies the tetrahedral Z site, as shown in Figure 3.1. 28

However, because of hydrogarnet substitution, OH- groups occupy some of the anion sites, where O4H4 tetrahedra substitute for some SiO4 tetrahedra, in an orderly fashion. Typically, the ratio of SiO4 : O4H4 is 2:1, because of under-bonding in the crystal structure (Friedrich et al. 2015). The ordering of small SiO4 and larger O4H4 tetrahedra in addition to Jahn-Teller distortion of the Mn3+ cation is reported as the reason for tetragonal symmetry.

Figure 3.1. Tetragonal henritermierite structure showing the linkages of the Y octahedra (blue), Z1 (light grey), and Z2 (dark grey) tetrahedra. The H atoms (orange) are bonded to O3 atoms (pink) when the Z2 site is vacant. The O1 (yellow) and O2 (purple) atoms are also shown. The X1 and X2 dodecahedral sites are omitted for clarity. The structure is projected down [100] and the O – H bonds are clearly observed.

Spessartine, ideally Mn3Al2Si3O12, is a member of the pyralspite series and is typically cubic, with space group Ia3d. However, substitution on the anion site, namely by OH- groups or F- atoms can occur. Substitution on the anion (Z) site is most common in grossular, andradite and uvarovite, where Ca occupies the dodecahedral X site, and Al, Fe3+ or Cr3+ occupies the Y site, respectively.

Substitution on the anion site is rare in pyrope, almandine and spessartine, where the X site is occupied by smaller cations: Mg, Fe2+ and Mn2+, respectively and the Y site is occupied by Al (Boiocchi et al. 2012). When this substitution occurs, the rare (OH,F)-bearing

2+ spessartine, Mn 3Al2[(SiO4)2(O4H4,F4)1]Σ3 hydrogarnet is formed, which as discussed above, has tetragonal symmetry, space group I41/acd, and is shown in Figure 3.2. In this case, the tetragonal symmetry is believed to originate from the anion substitution.

Birefringence in cubic garnets was reported over a century ago (Brewster 1853, Mallard

1876), but the origin remains uncertain. Some almandine, grossular, spessartine, andradite, uvarovite and hydrogarnet samples are anisotropic under cross-polarized light, which may indicate that they are not optically cubic (Allen and Buseck 1988, Brown and Mason 1994,

Deer et al. 1982, Rossman and Aines 1991). Several different reasons have been given for the birefringence, but the dominant one appears to be cation order at the X and Y sites that causes a symmetry reduction (Akizuki 1984, Allen and Buseck 1988). Other suggested reasons for birefringence in cubic garnets include plastic deformation, rare earth element substitution into the crystal structure, strain caused by lattice mismatch, cation ordering at the octahedral site, and incorporation of OH- groups into the crystal structure (Allen and Buseck 1988). This study examines the crystal chemistry of a henritermierite sample and an (OH,F)-rich spessartine 30

sample using electron-probe microanalysis (EPMA), single-crystal X-ray diffraction

(SCXRD), and synchrotron high-resolution power X-ray diffraction (HRPXRD).

Figure 3.2. Tetragonal (OH,F)-spessartine structure showing the linkages of the Y octahedra (green), Z1 (light grey), and Z2 (dark grey) tetrahedra. The O1 (yellow) and O2 (purple), O3 (pink), and F (teal) atoms are also shown. The X1 and X2 dodecahedral sites are omitted for clarity. The structure is projected down [001]; open channels parallel to the c axis occur when the Z2 site is vacant.

3.2 Sample Description and Experimental Techniques

The henritermierite sample is from Wessels Mine X, Kalahari Manganese Field, North

Cape, South Africa. This sample was obtained from the Royal Ontario Museum (ROM

#M54234) and is comprised of orange-brown euhedral crystals, less than 1 to 3 mm in diameter, that exhibit birefringence when viewed under the optical microscope, a shown in

Figure 3.3.

The (OH,F)-bearing spessartine sample is from Tongbei, near Yunxiao, Fujian Province,

China. This sample contains numerous crystals that are 1-3 mm in diameter, and transparent orange in color. Under cross-polarized light of the optical microscope, the crystals show birefringence and contain lamellar features, as seen in Figure 3.4. However, under plane- polarized light and in back-scattered electron (BSE) images, inhomogeneous features were not observed.

3.2.1 Electron-probe Microanalysis (EPMA )

Quantitative chemical compositions and back scattered electron (BSE) images were collected with a JEOL JXA-8200 WD-ED electron-probe microanalyzer (EPMA), for both samples. The JEOL operating program on a Solaris platform was used for ZAF correction and data reduction. The wavelength-dispersive (WD) operating conditions were 15 kV accelerating voltage, 20 nA beam current and 5 µm beam diameter. The instrument was calibrated to detect cations of interest using the following mineral standards: almandine-pyrope (Mg Kα), grossular

(Ca Kα), almandine (Fe Kα, Al Kα, Si Kα), rutile (Ti Kα), spessartine (Mn Kα), chromite (Cr

Kα) and fluorapatite (F Kα).

100 µm

Figure 3.3. (a) Plane-polarized light (PPL) and (b, c) cross-polarized light (XPL) images of a thick section of henritermierite. The XPL images show birefringence, but the PPL, XPL and BSE images (Figure 3.5) do not show any significant inhomogeneous features.

100 µm

Figure 3.4. (a) Plane-polarized light (PPL) and (b, c) cross-polarized light (XPL) images from a 80 µm-thick thin section of (OH,F)-bearing spessartine. Image c has been rotated to the same orientation as images a, b. The XPL images show birefringence and some lamellar features, but the PPL, and BSE images (Figure A2) do not show any significant inhomogeneous features.

After analyses were completed, oxide proportions from ten points across each sample were used to calculate atoms per formula unit (apfu) and chemical formulae. Atoms per formula unit was calculated on the assumption that the X and Y cations should sum to 5, following the 3:2:3 stoichiometry of garnet, while also considering full site occupancies and charge neutrality. Because the electron-probe detects iron as Fe, stoichiometric constraints were used to calculate the proportion of Fe2+ and Fe3+ cations present in each sample by converting FeO to Fe2O3 by a factor of 1.1113. For the analysis of henritermierite, all of the

FeO was converted to Fe2O3, but only a portion of FeO was converted for (OH,F)-spessartine.

In each sample, the water content was calculated by difference, on the assumption that the sum of the Z site apfu should be 3, calculated by (Si (apfu) + (OH,F)/4 = 3). The oxide wt.% and atoms per formula unit are provided in Tables 3.1a and b. The composition of the samples is similar to those reported for other henritermierite and (OH,F)-bearing spessartine samples.

3.2.2 Single Crystal X-ray Diffraction (SCXRD)

Each sample was studied with single crystal X-ray diffraction (SCXRD) with the

Nonius Kappa APEX2 CCD on a diffractometer using a Bruker Nonius FR591 Rotating Anode with graphite monochromatized Mo-Kα radiation in the Department of Chemistry at the

University of Calgary. A small crystal fragment (approximately 0.05 x 0.05 x 0.06 mm3) was mounted on the end of a glass fiber (0.05 mm in diameter) with epoxy and placed on a goniometer. The generator setting was 50 kV and 36 mA and the detector-to-crystal distance was fixed at 35 mm. A total of ten frames were collected for unit-cell determination. The scan settings were 1° rotation per frame (total rotation = 10°) and 22 seconds of X-ray exposure time per frame. After obtaining satisfactory unit-cell parameters and mosaicity values (less than 1°), a complete data set was collected using a 2° per frame rotation with exposure of 42-122 34

seconds per frame. The diffraction spots were measured in full, scaled with SCALEPACK, corrected for Lorentz-polarization, and integrated using the Nonius program suite DENZO-

SMN (Otwinowski and Minor 1997).

The space group I41/acd was obtained based on systematic absence of reflections and structure factor statistics. Full-matrix least-squares refinements were carried out with the

SHELXL program using neutral atom scattering factors (Sheldrick 2015) and the WinGX platform (Farrugia 2012). For henritermierite, the starting structural model was from

Armbruster et al. (2001). For (OH,F)-bearing spessartine, the starting structural model was from Boiocchi et al. (2012).

Anisotropic displacement parameters were used for all atoms in (OH,F)-bearing spessartine. For henritermierite, only the Si2 and H3 sites were refined using isotropic displacement parameters. For the SCXRD structure refinements of both minerals, the site occupancy factors (s.o.f.s) were refined together with the isotropic displacement parameters in a stepwise manner; the resulting s.o.f.s were fixed while the anisotropic displacement parameters were refined.

3.2.3 Synchrotron high-resolution powder X-ray diffraction

Both samples were studied using HRPXRD at beamline 11-BM, Advanced Photon

Source, Argonne National Laboratory. A small crystal fragment (about 2 mm in diameter) of each sample was crushed under methanol to a fine powder using a corundum mortar and pestle.

The crushed sample was loaded into a Kapton capillary (0.8 mm internal diameter) and rotated during the experiment at a rate of 90 rotations per second. The data were collected at 296 K to a maximum 2θ of ~50 ° with a step size of 0.001 ° and a step time of 0.1 seconds per step. The

HRPXRD patterns were collected with a unique multi-analyzer assembly consisting of 12 35

independent silicon (111) crystal analyzers and LaCl3 scintillation detectors that reduce the angular range to be scanned and allow rapid data acquisition. Silicon (NIST 640c) and alumina

! ! (NIST 676a) standards (ratio of !!Si: !!Al2O3 by weight) were used to calibrate the instrument and refine the monochromatic wavelength used in the experiment (described in Table 3.3). 36

Table 3.1a. Electron-probe microanalysis (EPMA) and atoms per formula unit (apfu) for henritermierite from Wessels Mine X, Northern Cape, South Africa (ROM #M54234). 1 2 3 4 5 6 7 8 9 10 ! †

SiO2 wt% 24.55 24.56 24.65 24.64 24.60 24.70 24.65 24.39 24.52 24.58 24.58(9)

TiO2 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00(0)

Al2O3 0.45 0.46 0.43 0.41 0.43 0.46 0.46 0.46 0.48 0.48 0.45(2)

Cr2O3 0.00 0.04 0.02 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.01(2) †† Fe2O3 0.41 0.41 0.37 0.33 0.34 0.65 0.46 0.52 0.75 1.22 0.55(7)

Mn2O3 31.81 31.26 31.60 31.69 32.01 31.39 31.69 31.48 31.37 30.61 31.49(8) MgO 0.03 0.04 0.02 0.01 0.00 0.02 0.06 0.04 0.03 0.01 0.03(2) CaO 35.03 34.44 34.73 34.78 35.20 34.85 34.94 34.80 35.00 34.67 34.84(1)

Na2O ------F 0.00 0.00 0.08 0.00 0.00 0.10 0.00 0.09 0.00 0.00 0.03(4) ††† H2O 7.82 7.44 7.52 7.58 7.86 7.55 7.72 7.73 7.81 7.55 7.66(5) Σ 100.09 98.65 99.42 99.42 100.45 99.72 99.97 99.52 99.98 99.12 99.63(2) Ca apfu 2.996 2.995 2.997 2.999 3.000 2.997 2.994 2.995 2.997 2.999 2.997(2) Mg 0.004 0.005 0.003 0.001 0.000 0.003 0.007 0.005 0.003 0.001 0.003(2) ΣX 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 Mn3+ 1.933 1.931 1.937 1.941 1.938 1.918 1.929 1.925 1.908 1.881 1.924(8) Al 0.042 0.044 0.041 0.039 0.040 0.043 0.043 0.044 0.045 0.045 0.043(2) Fe3+ 0.025 0.025 0.023 0.020 0.020 0.039 0.027 0.032 0.045 0.074 0.033(7) Cr3+ 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.001(1) Ti4+ 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000(0) ΣY 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 Si 1.960 1.993 1.985 1.983 1.957 1.983 1.971 1.959 1.959 1.984 1.973(4)

F4 0.000 0.000 0.005 0.000 0.000 0.007 0.000 0.006 0.000 0.000 0.002(7)

H4 1.040 1.000 1.010 1.017 1.043 1.011 1.029 1.035 1.041 1.016 1.025(7) ΣZ 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 1 3+ 3+ †! represents the average of ten data points and gives an average formula of Ca3.00{Mn 1.92Al0.04Fe 0.03}Σ2.00[(SiO4)1.97(O4H4)1.03]Σ3.00 ††FeO was converted to Fe2O3 by a factor of 1.1113 †††H2O was calculated from Si + (OH,F)/4 = 3. apfu was calculated on the basis of X + Y = 5. 37

Table 3.1b. Electron-probe microanalysis (EPMA) and atoms per formula unit (apfu) for (OH,F)-bearing spessartine from Fujian Province, China. 1 2 3 4 5 6 7 8 9 10 !†

SiO2 wt% 31.36 32.25 32.23 32.29 31.93 32.14 31.93 31.99 32.13 32.18 32.04(7)

TiO2 0.01 0.06 0.07 0.06 0.06 0.06 0.06 0.08 0.06 0.07 0.06(2)

Al2O3 20.37 20.31 20.21 20.11 20.36 20.36 20.44 20.37 20.32 20.49 20.33(1)

Cr2O3 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00(0)

Fe2O3 0.72 0.74 0.89 0.89 0.78 0.72 0.64 0.70 0.70 0.58 0.74(0) FeO 1.40 2.17 2.25 2.03 2.14 2.10 2.10 2.19 2.12 2.26 2.08(8) MgO 0.00 0.02 0.02 0.04 0.03 0.04 0.03 0.00 0.02 0.03 0.02(1) CaO 0.50 0.46 0.45 0.47 0.46 0.49 0.46 0.43 0.47 0.45 0.46(2) MnO 41.49 40.71 40.65 40.57 40.88 40.81 40.88 40.83 40.71 40.77 40.83(5)

Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00(0) F 2.02 1.65 1.71 1.72 1.86 1.77 1.84 1.72 1.94 1.54 1.78(4) †† H2O 2.33 2.33 1.94 1.78 2.13 2.01 2.12 2.12 1.87 2.12 2.08(8) -F≡O 0.85 0.69 0.72 0.72 0.78 0.75 0.77 0.73 0.82 0.65 0.75(6) Σ 99.35 99.64 99.70 99.24 99.86 99.78 99.73 99.71 99.52 99.82 99.64 Mn2+ apfu 2.862 2.809 2.806 2.814 2.811 2.810 2.814 2.813 2.812 2.804 2.815(7) Fe2+ 0.095 0.148 0.153 0.139 0.145 0.143 0.143 0.149 0.144 0.153 0.141(7) Ca 0.044 0.040 0.039 0.042 0.040 0.043 0.040 0.038 0.041 0.039 0.040(2) Mg 0.000 0.003 0.002 0.005 0.004 0.005 0.004 0.000 0.002 0.003 0.003(2) ΣX 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 Al 1.954 1.951 1.941 1.942 1.948 1.951 1.958 1.952 1.954 1.961 1.951(6) Fe3+ 0.044 0.046 0.055 0.055 0.048 0.044 0.039 0.043 0.043 0.035 0.045(6) Ti4+ 0.001 0.004 0.004 0.004 0.004 0.004 0.004 0.005 0.004 0.004 0.004(1) Cr3+ 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000(0) ΣY 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 Si 2.554 2.628 2.626 2.645 2.592 2.613 2.595 2.602 2.620 2.614 2.609(5)

F4 0.130 0.106 0.110 0.111 0.120 0.114 0.118 0.111 0.125 0.099 0.114(6)

H4 0.317 0.266 0.264 0.243 0.288 0.273 0.287 0.287 0.255 0.287 0.277(4) ΣZ†† 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 2+ 3+ †! represents the average of the ten data points and gives an average formula of (Mn2.82Fe 0.14Ca0.04)Σ3.00{Al1.95Fe 0.05}Σ2.00[(SiO4)2.61(OH)0.28F0.11]Σ3.00. ††H2O was calculated from Si + (OH,F)/4 = 3. apfu was calculated on the basis of X + Y = 5. 38

Table 3.2. SCXRD data for (a) henritermierite and (b) (OH,F)-bearing spessartine. (a) Henritermierite † This Study Armbruster et al. (2001) Chemical Formula Ca3(Mn1.94Al0.06)Σ2[(SiO4)2.02(O4H4)1]Σ3 Unit-cell parameters (Å) a = b = 12.4908(2) a = b = 12.489(1) c = 11.9092(2) c = 11.909(1) c/a 0.9534 0.9536 Volume (Å3) 1858.07(5) 1857.5 a (pseudo cubic) (Å) 12.2938 12.293 Density (calc.) (Mg m-3) 3.4431 Absorption coefficient (mm-1) 4.63 F(000) 1901 Crystal size (mm) 0.05 x 0.05 x 0.06 0.1 x 0.2 x 0.18 θ range (°) 4.13-39.98 ≤ 40 No. of reflections collected 17620 4235 No. of independent reflections 1401 (Rint = 0.0294) 1167 No. of observed reflections 1340 [I > 2σ(I)] -1 (sin θ/λ)max (Å ) 0.904 Refinement method Full-matrix least-squares on F2 No. of data 1401 S (goodness of fit on F2) 1.209 1.224 Final R indices R1 = 0.0204, wR2 = 0.0428 R1 = 0.0162, wR2 = 0.0390 [F2 > 2σ(F2)] R indices (all data) R1 = 0.0227, wR2 = 0.0432 -3 Δρmax (e Å ) 0.44 (0.80 Å from O1) -3 Δρmin (e Å ) -1.36 (0.00 Å from Mn) (b) (OH,F)-bearing spessartine † This Study Boiocchi et al. (2012) Chemical Formula Mn3Al2[(SiO4)2.61(O4H4)0.23(F4)0.25)]Σ3 Unit-cell parameters (Å) a = b = 11.6446(2) a = b = 11.6347(3) c = 11.6548(2) c = 11.6449(3) c/a 1.0009 1.0009 Volume (Å3) 1580.35(5) 1576.3(1) a (pseudo cubic) (Å) 11.648 11.638 Density (calc.) (Mg m-3) 4.076 4.085 Absorption coefficient (mm-1) 5.43 F(000) 1869 Crystal size (mm) 0.07 x 0.08 x 0.09 0.15 x 0.30 x 0.42 θ range (°) 4.28-62.81 1-55 No. of reflections collected 50335 24602 No. of independent reflections 3235 (Rint = 0.0364) 2473 (Rint = 0.0360) No. of observed reflections 2665 [I > 2σ(I)] -1 (sin θ/λ)max (Å ) 1.252 Refinement method Full-matrix least-squares on F2 No. of data, 3235 S (goodness of fit on F2) 1.232 Final R indices R1 = 0.0290, wR2 = 0.0743 [F2 > 2σ(F2)] R indices (all data) R1 = 0.0387, wR2 = 0.0775 R1 = 0.0314 -3 Δρmax (e Å ) 0.68 (0.40 Å from O3) 0.83 -3 Δρmin (e Å ) -1.65 (0.00 Å from Si2) †Temperature = 296(2) K, λ (Mo Kα) = 0.71073 Å, Z = 8, tetragonal space group I41/acd 39

3.3 Results

EPMA results for both minerals are given in Tables 3.1a and b. Details of the SCXRD data collection, processing and refinement are given in Table 3.2. Details of the HRPXRD data collection and Rietveld refinement statistical indicators are given in Table 3.3. The atom coordinates, s.o.f.s, and equivalent isotropic displacement parameters are given in Table 3.4.

Anisotropic displacement parameters are given in Table 3.5. Selected bond distances and angles are given in Table 3.6, and bond valence sums (BVS) are given in Table 3.7.

Table 3.3. HRPXRD data and Rietveld refinement statistical indicators Henritermierite (OH,F)-bearing spessartine a (Å) 12.49075(5) 11.64463(1) c (Å) 11.90921(5) 11.65481(2) c/a 0.95344 1.0009 V (Å3) 1858.06(1) 1580.361(4) Reduced χ2 4.570 1.771 R(F2) 0.0693 0.0403 wRp 0.1398 0.0631

Nobs 2215 1827 λ (Å) 0.41374(2) 0.41383(2) Data points 47995 47992 R(F2) = overall R-structure factor based on observed and calculated structure amplitudes. 2θ range = 2-50°.

Table 3.4. Atom coordinates, s.o.f.s and equivalent isotropic displacement parameters for (a) henritermierite and (b) (OH,F)-bearing spessartine. (a) Henritermierite 2 Site s.o.f.s apfu x y z Ueq (Å ) Ca1 = X1 16e 1.0 2 Ca 0.3617(1) 0 1/4 0.007(1) Ca2 = X2 8b 1.0 1 Ca 0 1/4 1/8 0.009(1) Mn = Y 16c 0.970(4)† 1.94 Mn2+ + 0.06 0 0 0 0.006(1) Al Si1 = Z1 16e 1.0 2 Si 0.1159(1) 0 1/4 0.005(1) Si2 = Z2 8a 0.006(5) 0.02 Si + 0.98 ☐ 1/2 1/4 1/8 0.012(3) O1 32g 1.0 4 O 0.2948(1) 0.7184(1) 0.0966(1) 0.008(1) O2 32g 1.0 4 O 0.1599(1) 0.5549(1) 0.0538(1) 0.008(1)

O3 32g 1.0 4 O or 1 O4 0.4429(1) 0.3599(1) 0.0216(1) 0.009(1) H3 32g 1.0 4 H or 1 H4 0.4292(16) 0.3520(17) 0.0806(19) 0.042(5)

(OH,F)-bearing spessartine Mn1 = X1 16e 1.0 2 Mn 0.3754(1) 0 1/4 0.008(1) Mn2 = X2 8b 1.0 1 Mn 0 1/4 1/8 0.008(1) Al = Y 16c 1.0 2 Al 0 0 0 0.005(1) Si1 = Z1 16e 0.820(4) 1.64 Si + 0.36 ☐ 0.1255(1) 0 1/4 0.004(1) Si2 = Z2 8a 0.972(5) 0.97 Si + 0.03 ☐ 1/2 1/4 1/8 0.005(1) O1 32g 0.851(4) 3.40 O + 0.60 ☐ 0.2979(1) 0.7143(1) 0.0977(1) 0.006(1) O2 32g 0.77(8) 3.08 + 0.92 ☐ 0.1523(3) 0.5356(2) 0.0471(2) 0.006(1) O3 32g 1.0 4 O 0.4526(1) 0.3480(1) 0.0354(1) 0.007(1)

F11 32g 0.149(4) 0.60 F or 0.15 F4 0.3069(7) 0.7319(6) 0.1058(6) 0.010(1)

O22 32g 0.23(8) 0.92 O or 0.23 O4 0.1470(9) 0.5206(6) 0.0537(8) 0.007(1)

3.3.1 Henritermierite

Electron-probe microanalysis (EPMA) gives a chemical formula of

Ca3(Mn1.94Al0.04Fe0.02)Σ2[(SiO4)1.98(O4H4)1.02]Σ3, as presented in Table 3.1a. The corresponding formula from the SCXRD structure refinement is Ca3(Mn1.94Al0.06)Σ2[(SiO4)2.02(O4H4)1]Σ3, which is similar to the ideal henritermierite formula: Ca3Mn2[(SiO4)2(O4H4)1]Σ3. As these two chemical formulas are in good agreement, the SCXRD structure refinement produced reasonable results.

All atom sites are fully occupied in henritermierite, except for the apparent vacant Si2 site, as seen in Table 3.4. The unit-cell parameters for henritermierite are a = b = 12.4908(2) Å and c =

11.9092 Å. This is a significant deviation from cubic symmetry, giving c/a = 0.9534, instead of the expected ratio for cubic minerals of c/a = 1.000.

The O3 and H3 sites are fully occupied, in contrast to work done by Armbruster et al.

(2001), where the O3 position was split between the O3 and O33 positions. The Ueq value for

O3 is similar to those for O1 or O2, and the Ueq for the Si2 site is large, indicating a site vacancy, confirmed by the small site occupancy factor, given in Table 3.4. The occurrence of an unsplit

O3 site, along with the H3 atom confirms the presence of O4H4 tetrahedra.

The bond angles and distances reported in this study, given in Table 3.6a, are nearly the same as those obtained by Armbruster et al. (2001), if their reported O33 site is omitted. The under-bonded O3 atom is charge balanced by forming a bond with a hydrogen atom, where O3-

H3 = 0.73(2) Å, as presented in Tables 3.6a and 3.7a. This bond length is shorter than the expected 1 Å of a typical O – H bond and is justified by formation of an H3---O2 bond, with length = 2.2 Å. The bond valence sum indicates that the O2 atom is under-bonded because of the

Mn-O2 Jahn-Teller elongation.

Table 3.5. Anisotropic displacement parameters (Å2) for (a) henritermierite and (b) (OH,F)- bearing spessartine.

(a) Henritermierite

U11 U22 U33 U23 U13 U12 Ca1 0.006(1) 0.007(1) 0.008(1) -0.001(1) 0 0 Ca2 0.009(1) 0.009(1) 0.007(1) 0 0 -0.003(1) Mn3+ 0.006(1) 0.006(1) 0.005(1) -0.001(1) -0.001(1) 0.001(1) Si1 0.005(1) 0.006(1) 0.005(1) 0.000(1) 0 0 O1 0.010(1) 0.007(1) 0.006(1) -0.001(1) 0.000(1) -0.001(1) O2 0.008(1) 0.008(1) 0.008(1) 0.001(1) 0.001(1) -0.002(1) O3 0.010(1) 0.008(1) 0.009(1) -0.001() 0.003(1) -0.002(1)

(b) (OH,F)-bearing spessartine U11 U22 U33 U23 U13 U12 Mn1 0.005(1) 0.010(1) 0.009(1) -0.002(1) 0 0 Mn2 0.009(1) 0.009(1) 0.005(1) 0 0 -0.002(1) Al 0.005(1) 0.004(1) 0.005(1) 0.000(1) 0.000(1) 0.000(1) Si1 0.004(1) 0.005(1) 0.005(1) 0.000(1) 0 0 Si2 0.005(1) 0.005(1) 0.004(1) 0 0 0 O1 0.007(1) 0.006(1) -0.001(1) 0.000(1) 0.000(1) -0.001(1) O2 0.005(1) 0.005(1) 0.007(1) 0.001(1) 0.000(1) -0.001(1) O3 0.008(1) 0.006(1) 0.007(1) 0.001(1) -0.001(1) 0.000(1) O22 0.007(1) 0.006(2) 0.009(1) 0.001(1) -0.001(1) 0.000(1) F11 0.009(2) 0.011(2) 0.009(1) -0.002(1) 0.000(1) -0.002(1)

3+ The Mn - O6 octahedra contain Jahn-Teller distortions in henritermierite where the average = 2.0219(3) Å, because of an elongation of the Mn – O2 bond, length =

2.2068(5) Å, given in Table 3.6a. The observed Jahn-Teller elongation is less than that in

2+ 3+ hausmannite spinel, ideally Mn Mn 2O4, discussed in Chapter 4 of this thesis.

The average < Si1 – O > = 1.6432(4) Å, is the same as many anhydrous cubic garnets with no significant Si vacancies. A vacancy in the Si2 atom site indicates the occurrence of

O4H4 tetrahedra, and the lowering of symmetry from cubic to tetragonal arises from ordering of

3+ [SiO4] and [O4H4] tetrahedra, triggered by O2 – Mn – O2 Jahn-Teller elongation (Armbruster et al. 2001). Henritermierite and hausmannite spinel occur as an intergrowth, observed in a BSE image, shown in Figure 3.5. Hausmannite is thought to be an original mineral from which 43

henritermierite was formed by a reaction that includes quartz, SiO2, calcite, CaCO3, and H2O. A possible formation reaction is:

! !Mn3O4 + 2SiO2 + 3 CaCO3 + 2H2O + 0.165O2 = Ca3Mn2[(SiO4)2(O4H4)1]Σ3 + 3CO2.

It has also been proposed that henritermierite from the Kalahari Manganese Field forms as a hydrothermal reaction product of the original braunite-rich manganese ores (Cairncross et al.

1997). Crystal structure refinements show that cation vacancies occur in garnet minerals and are well studied in cubic hydrogarnets (Antao 2015, Basso et al. 1984). Si deficiencies of about 5 - 6

% indicate minor substitution of (O4H4) groups for SiO4 groups. If (O4H4) substitution occurs, the site occupancy factor of the tetrahedral Z site is less than 1, and the unit-cell edge a increases, causing Z – O to increase and Y – O to decrease in length, simultaneously (Antao 2015).

Figure 3.5. BSE image showing an intergrowth of henritermierite (dark grey) and hausmannite (white).

Table 3.6. Selected bond distances (Å) and angles (°) for (a) henritermierite and (b) (OH,F)- bearing spessartine. (a) Henritermierite (b) (OH,F)-bearing spessartine This Armbruster This Boiocchi

Study et al. (2001) Study et al. (2012) Ca1 - O1 x2 2.4773(5) 2.476(1) Mn1-O1 x2 2.256(1) 2.251(1) Ca1 - O2 x2 2.4504(5) 2.450(1) Mn1-O2 x2 2.260(3) 2.257(1) Ca1 - O2 x2 2.4504(5) 2.451(1) Mn1-O2 x2 2.422(3) 2.407(1) Ca1 - O3 x2 2.4484(6) 2.445(1) Mn1-O3 x2 2.4148(5) 2.418(1) Ca1 - O33 x2 - 2.66(3) Mn1-O22 x2 2.173(11) - Mn1-O22 x2 2.315(9) - Ca2 - O1 x4 2.6157(5) 2.614(1) Mn1-F11 x2 2.173(9) - Ca2 - O3 x4 2.3328(5) 2.334(1) Ca2 - O33 x4 - 2.33(3) Mn2-O1 x4 2.411(1) 2.416(1) Mn2-O3 x4 2.2583(5) 2.258(1) Mn - O1 x2 1.9513(5) 1.952(1) Mn2-F11 x4 2.269 (8) - Mn - O2 x2 2.2068(5) 2.206(1) Mn - O3 x2 1.9075(5) 1.904(1) Al-O1 x2 1.906(1) 1.896(1) Mn - O33 x2 - 2.04(3) Al-O2 x2 1.902(3) 1.901(1) Al-O3 x2 1.8993(5) 1.903(1) Si1 - O1 x2 1.6566(5) 1.657(1) Al-O22 x2 1.838(10) - Si1 - O2 x2 1.6297(5) 1.630(1) Al-F11 x2 1.818(8) -

Si2 - O3 x4 1.9773(6) 1.981(1) Si1-O1 x2 1.6440(9) 1.642(1) Si2 - O33 x4 - 1.70(2) Si1-O2 x2 1.633(2) 1.645(1) Si1-O22 x2 1.639(1) - 2.1907(2) 2.191(1) Si1-F11 x2 1.878(5) - Mn - O1 - Si1 x1 132.99(2) - Mn - O2 - Si1 x1 134.39(3) - Si2-O3 x4 1.6427(4) 1.634(1) Mn - O3 - Si2 x1 125.86(3) - Si2-O33 x4 - 1.838(4) O3 - H3 x1 0.73(2) 0.75(3) Si2 - H3 x1 1.64(2) - 2.054(1) 2.052(1) H3 - O2 x1 2.198 2.21(3) Al-O1-Si1 x1 133.22(7) 133.4 O3 - O2 x1 2.773 2.763(1) Al-O2-Si1 x1 133.7(1) 133.2 O3 - O3 x2 3.294 3.301(1) Al-O3-Si2 x1 133.45(3) 133.6 O3 - O3 x1 3.094 3.095(2) Al-O33-Si2 x1 - 125.1 O3 - H3 - O2 x1 136.4 134(3) O3 - H3 - O3 x1 140.3 138(3) For henritermierite, Armbruster et al. (2001) split the O-atom site into O3 and O33. The O33 site represents the O atom of the SiO4 tetrahedra. There are no significant differences between this study and the study by Armbruster et al. (2001).

Table 3.7. Bond valence sums (v.u.) for (a) henritermierite and (b) (OH,F)-bearing spessartine. (a) Obtained with SCXRD data for henritermierite Si1 Si2 Ca1 Ca2 Mn3+ Σ O1 0.888 x 2 0.244 x 2 0.168 x 4 0.569 x 2 1.869 O2 0.955 x 2 0.263 x 2 0.285 x 2 0.263 x 2 1.766 O3 0.264 x 2 0.361 x 4 0.641 x 2 1.266 H3≡O† ≅ 0.9 x 4 Σ 3.686 3.6 2.066 2.116 2.990

(b) (OH,F)-bearing spessartine Si1 Si2 Mn1 Mn2 Al Σ O1 0.805 x 2 0.241 x 2 0.146 x 4 0.392 x 2 1.584 O2 0.751 x 2 0.216 x 2 0.359 x 2 O2 0.139 x 2 1.465 O3 0.951 x 4 0.185 x 2 0.260 x 4 0.470 x 2 1.866 O22 0.137 x 2 0.082 x 2 O22 0.056 x 2 0.127 x 2 0.402 F11 0.067 x 2 0.042 x 2 0.029 x 4 0.072 x 2 0.210 Σ 3.523 3.803 1.920 1.741 2.841 †If an Si2 atom is present, the O atom may occupy the H3 position with O3 vacant, such that each O contributes ≅ 0.9 v.u. to Si2.

3.3.2 (OH,F)-bearing spessartine

Electron-probe microanalysis (EPMA) gives a chemical formula of

2+ 3+ {Mn2.82Fe 0.14Ca0.04}Σ3(Al1.95Fe 0.05)Σ2[(SiO4)2.61(O4H4)0.28(F4)0.11]Σ3, as presented in Table 3.1b.

The corresponding formula from the SCXRD structure refinement, presented in Table 3.3b, is

Mn3Al2[(SiO4)2.61(O4H4)0.23(F4)0.15]Σ3, which is similar to the ideal (OH,F)-bearing spessartine formula: Mn3Al2[SiO4)2(O4H4,F4)1]Σ3. As these two chemical formulas are in close agreement, the SCXRD structure refinement produced reasonable results.

The tetragonal unit-cell parameters obtained for (OH,F)-bearing spessartine are a = b =

11.6446(2) Å and c = 11.6548 Å. This is a much smaller deviation from cubic symmetry than observed in henritermierite, as c/a = 1.0009, very similar to the ratio of c/a = 1.000 expected for cubic minerals. The complete HRPXRD pattern for the (OH,F)-bearing spessartine sample is shown in Figure 3.6. Expanded sections of the pattern show that the 200 reflection peak is a single peak, but the 004 and 400 reflections occur as split peaks, because of the tetragonal symmetry (Figure 3.6b, c). The splitting of these characteristic cubic reflections is an indicator of tetragonal symmetry in garnets (Heinemann et al. 1997, Parise et al. 1996), but has also been attributed to the presence of two phases (Antao 2013a,b,c, Antao and Hassan 2010).

HRPXRD patterns for cubic spessartine do not contain the 200 reflection peak, and this peak is not a strong peak in tetragonal (OH,F)-bearing spessartine, but is strong in henritermierite, as shown in Figure 3.7. Therefore, the presence or absence of the 200 reflection may be used to differentiate tetragonal and cubic garnets. Henritermierite contains a much stronger 200 reflection, and exhibits more violations of cubic symmetry, as seen by the number of split peaks. The bond angles and distances reported in this study are similar to those obtained by Boiocchi et al. (2012), but significant differences are observed, as presented in Table 3.6b. 48

Figure 3.6. Complete HRPXRD pattern for tetragonal (OH,F)-spessartine. (a) 0 – 50 ° 2θ range. The difference curve (Iobs – Icalc) is shown at the bottom. Short vertical lines indicate allowed reflection positions. Expanded traces of the (b) 200 and (c) 004 and 400 peaks are also given. The presence of the 200 reflection, along with the splitting of the 004 and 400 reflections indicates tetragonal symmetry.

Figure 3.7. Comparison of experimental traces for (a) cubic (Ia3d) spessartine from Colorado (Smyth et al. 1990), (b) tetragonal (I41/acd) (OH,F)-spessartine and (c) tetragonal (I41/acd) henritermierite. The 200 reflection is absent in (a) but is observed in the tetragonal garnets (b, c).

The Z1 atom site contains some vacancies, but the Z2 site is fully occupied, as presented in Table 3.4b. The O3 site is fully occupied, unlike that of (OH,F)-spessartine (Boiocchi et al.

2012) and henritermierite studied by Armbruster et al. (2001) where the O3 position was split into O3 and O33. The other two O atom positions are split into the O1 and F11 positions and O2 and F22 positions. The F11 site and O22 sites contain F- atoms and OH- groups, respectively.

Because of the size difference of the substituent atoms occupying these sites (F- (1.31 Å), OH-

(1.35 Å), and O2- (1.38 Å)), the Z - F11 distance is the largest, followed by Z1 - O22 and Z1 - O, as presented in Table 3.6b.

As the Z1 and Z2 sites are in tetrahedral coordination, the vacant Z site is expected to have a large average distance. However, this is not the case, as this study gave

O> = 1.639(1) Å and Z2 – O3 = 1.6427(4) Å, which are nearly the same as each other, and are similar to those in anhydrous cubic spessartine with no significant Si-atom vacancies (Antao and

Round 2014, Merli et al. 1995, Novak and Gibbs 1971). In the study by Boiocchi et al. (2012), they obtained = 1.644(1) Å and = 1.634(1) Å, which are significantly different from the results of this study. When the Z1 site is vacant, the tetrahedron is comprised of O2H2 and F2, giving large Z1 – O22 and Z1 – F11 distances, with average =

1.847(4) Å, given in Table 3.6b. Although these significant differences between this study and the work of Boiocchi et al. (2012) exist, the mean bond distances are similar. The lowering in symmetry from cubic to tetragonal arises from ordering of [SiO4] and [(O2H2)F2] tetrahedra. This indicates a new possible end-member for (OH,F)-spessartine:

Mn3Al2[(SiO4)2(O2H2)0.5(F2)0.5]Σ3, which is currently unknown.

The bond-valence sums, presented in Table 3.7b, are close to expected values. They indicate that the O3 site is nearly charge balanced, but the O1 and O2 sites are under-bonded, 51

because they are part of split sites. The partially occupied F11 and O22 sites are also under- bonded, indicating that these contain monovalent F- and OH- ions, respectively. Site occupancy factors (s.o.f.s) agree with the chemical analysis.

It is likely that rhodochrosite, MnCO3, is an original mineral from which (OH,F)- spessartine formed as a hydrothermal reaction product that includes quartz, SiO2, corundum,

Al2O3, F2, and H2O. A possible formation reaction is:

3MnCO3 + Al2O3 + 2SiO2 + 0.5F2 + 0.5H2O = Mn3Al2[(SiO4)2(O2H2)0.5(F2)0.5]Σ3.

3.4 Conclusions

This study shows that most garnet-group minerals crystallize with cubic symmetry, but select rare garnet minerals crystallize with tetragonal symmetry. The known tetragonal garnet- group minerals are henritermierite, holtstamite and (OH,F)-bearing spessartine. The cause of tetragonal symmetry appears to be ordered substitution by OH-, F- and O2- anions. Although octahedral Y cations such as Mn3+ may trigger anion ordering through Jahn-Teller distortion, it is not a requirement in (OH,F)-rich spessartine. Tetragonal garnets containing OH- and F- anions were likely formed at lower temperatures than garnets in which the anion site is solely occupied by O2-. Tetragonal garnets have distinctly different X-ray diffraction patterns than those of cubic garnets, and can be differentiated by the presence of the low-angle 200 reflection, as well as peak-splitting, especially of the 004 and 400 peaks.

CHAPTER 4

CRYSTAL CHEMISTRY OF HAUSMANNITE

4.1 Introduction to Hausmannite

The structural properties of cubic spinel-group minerals were widely studied (e.g.,

O’Neill and Navrotsky 1983; Antao et al. 2005b; Antao et al. 2007). Hausmannite is a spinel- group mineral. Hausmannite, ideally Mn3O4, is a manganese oxide mineral with a distorted

[4] [6] tetragonal spinel structure. It is an AB2O4 oxide which has structural formula (A1-iBi) (B2- iAi)O4, where the superscript 4 and 6 represent the tetrahedral and octahedral sites, respectively.

A and B are cations with variable valence and i is an inversion parameter (Bosi et al. 2002).

[4] 2+ [6] 3+ End-member hausmannite has structural formula (Mn ) (Mn )2O4, where the tetrahedrally coordinated site is occupied by a divalent cation, and the octahedrally coordinated site is occupied by two trivalent cations; however, the valence of these two cation sites may vary.

Hausmannite is relatively uncommon, but has a wide field of stability. It is commonly found with other manganese oxide minerals in metamorphosed or hydrothermal manganese ores

(Frenzel 1976), but euhedral crystals are rare and have only been found in two known localities:

Ilmanau, Thuringia, Germany and the Kalahari manganese field in Northern Cape Province,

South Africa (Baron et al. 1998). It is typically ferrimagnetic, but a strongly magnetic sample has been discovered in the hydrothermally altered Kalahari manganese field (Gutzmer et al.

1995). Hausmannite may be initially confused with manganite, braunite or wolframite.

However, manganite is often columnar, twinned and replaced by pyrolusite, hausmannite has a much stronger anisotropy than braunite, and wolframite has different mineral associations than hausmannite (Frenzel 1976). 53

Under normal pressure conditions, hausmannite has two crystallographic forms: a tetragonal low-temperature form and a cubic high-temperature form (Frenzel 1976). The cubic- tetragonal transition occurs at 1170 °C, where the crystal lattice is deformed during this phase transition (Frenzel 1976). In the tetragonal hausmannite structure, the oxygen atoms form a tetragonally distorted cubic structure where the tetrahedral and octahedral sites, occupied by Mn cations are located within (Frenzel 1976). Tetragonal hausmannite is reported with space group

I41/amd (Frenzel 1976) and is shown in Figure 4.1, which was drawn using the program “Crystal

Maker”.

Figure 4.1. Ball and stick representation of the tetragonal hausmannite structure composed of Mn2+ (turquoise), Mn3+ (black) and oxygen (pink) atoms. The structure is projected down the c axis. The black dashed box represents the unit-cell outline.

Previously, synthetic hausmannite was prepared and refined using powder X-ray diffraction data (Satomi 1961) and was found to have unit-cell parameters a = b = 5.7691(4) Å and c = 9.4605(7) Å at 23 °C.

The structure of hausmannite from Langbån, Sweden was refined using single-crystal X- ray diffraction (SCXRD) data (Jarosch 1987). It was found to have unit-cell parameters a = b =

5.765(1) Å and c = 9.442(1) Å. The Mn1 – O bond was 2.040(1) Å x 4 and the Mn2 – O bonds were 1.930(1) Å x 4 and 2.282(1) Å x 2, where Mn1 and Mn2 represent Mn3+ and Mn2+, respectively.

Iron rich hausmannite, containing up to 11.3 wt% of Fe2O3 has unit-cell parameters that are smaller along the a axis and larger along the c axis. These parameters shrink along the a axis and expand along the c axis, proportional to increasing iron content (Baron et al. 1998).

In the Kalahari manganese field, the manganese ores are interbedded with the iron-rich

Hotazel formation and are found in erosional relict basins (Gutzmer et al. 1995). Within the high-grade Wessels type manganese ore, normal faults act as conduits for hydrothermal fluids which cause hematitization of manganese ores. This high-grade manganese ore is coarse grained and contains hausmannite, bixbyite, marokite and hematite (Gutzmer et al. 1995).

This study examines the crystal chemistry of a hausmannite sample from the Wessels

Mine X, Kalahari manganese field, Northern Cape Province, South Africa, using electron probe microanalysis (EPMA) and single-crystal X-ray diffraction (SCXRD).

4.2 Sample Description and Experimental Techniques

The sample used in this study is a black euhedral hausmannite crystal from a sample comprised dominantly of henritermierite (ROM # M54234) from Wessels X, Kalahari manganese field, Northern Cape Province, South Africa. The hausmannite was intergrown with 55

tetragonal garnet, henritermierite, ideally Ca3Mn2[(SiO4)2(O4H4)]Σ3 (Armbruster et al. 2001), discussed in Chapter 3 of this thesis.

4.2.1 Electron Probe Microanalysis (EPMA)

(Mg Kα, Fe Kα and Al Kα). The sample was loaded into a disc and secured with epoxy.

Following the setting of the epoxy, the sample was polished using a selection of progressively finer abrasives, beginning with coarse sand paper (P400) and ending with very fine diamond paste (1 µm), in order to achieve a surface free of scratches. After analyses were completed, the chemical composition was calculated on the basis of four oxygen atoms, with FeO converted to

Fe2O3 by a factor of 1.1113 and MnO converted to Mn2O3 by a factor of 1.1128; this analysis is given in Table 4.1.

4.2.2 Single Crystal X-ray diffraction (SCXRD)

The hausmannite sample was studied with single crystal X-ray diffraction (SCXRD) with the Nonius Kappa APEX2 CCD on a diffractometer using a Bruker Nonius FR591 Rotating

Anode with graphite monochromatized Mo-Kα radiation in the Department of Chemistry at the

was fixed at 35 mm. A total of ten frames were collected for unit-cell determination. The scan settings were 1° rotation per frame (total rotation = 10°) and 22 seconds of X-ray exposure time per frame. After obtaining satisfactory unit-cell parameters and mosaicity values (less than 1°), a complete data set was collected using a 2° per frame rotation with exposure of 42-122 seconds per frame. The diffraction spots were measured in full, scaled with SCALEPACK, corrected for

Lorentz-polarization, and integrated using the Nonius program suite DENZO-SMN (Otwinowski and Minor 1997).

The space group I41/amd was obtained based on systematic absence of reflections and structure factor statistics. Full-matrix least-squares refinements were carried out with the

SHELXL program using neutral atom scattering factors (Sheldrick 2015) and the WinGX platform (Farrugia 2012). The starting structural model was from Bosi et al. (2010). Anisotropic displacement parameters were used for all of the atoms in hausmannite. The site occupancy factors (s.o.f.s) were refined together with the isotropic displacement parameters in a stepwise manner and the resulting s.o.f.s were fixed while the anisotropic displacement parameters were refined. Details of the data collection, processing, and refinement are given in Table 4.2. The atom coordinates, s.o.f.s, and equivalent isotropic displacement parameters are given in Table

4.3. Anisotropic displacement parameters are given in Table 4.4. Selected bond distances and angles are given in Table 4.5. Bond-valence sums (BVS) are given in Table 4.6.

4.3 Results

Backscattered electron (BSE) images revealed the presence of hausmannite within the henritermierite sample. The BSE images (Figure 4.2) show hausmannite (white) that appears brighter than the henritermierite garnet (dark grey), indicating a higher atomic number. The BSE image does not display any zoning or other growth features. This polished crystal was used for 57

EPMA.Quantitative analyses were obtained from 9 points and the results are presented in Table

4.1. From the nine points analyzed with EPMA, the resultant chemical composition was

2+ 2+ 3+ {Mn 0.88Mg0.11Fe 0.01}Σ1Mn 2.00O4. Atoms per formula unit (apfu) were calculated on the basis of four oxygen atoms, after Bosi et al. (2010).

Single crystal X-ray diffraction (SCXRD) confirmed the presence of a single phase of hausmannite within the sample. The tetragonal hausmannite sample (space group I41/amd) had a and c unit-cell parameters of 5.7556(6) Å and 9.4426(11) Å, respectively. The goodness of fit

2 on F was found to be 1.144 and the R1 and wR2 was 0.0277 and 0.0559, respectively.

Selected bond distances include Mn1 – O1 = 1.9276(5) Å x 4, Mn1 – O1 = 2.2845(8) Å x 2 and Mn2 –O1 = 2.0361(8) x 4. A comparison of unit-cell parameters and bond distances of this study to previous studies on hausmannite can be found in Table 4.7. 58

Table 4.1. Electron-probe microanalysis (EPMA) and atoms per formula unit (apfu) of hausmannite from Wessels Mine X, North Cape, South Africa (ROM # M54234). Oxide Bosi et al. Point 1 Point 2 Point 3 Point 4 Point 5 Point 6 Point 7 Point 8 Point 9 !1 (wt%) (2010)2 MnO 27.30 26.56 27.15 27.02 27.41 27.49 27.36 27.36 27.74 27.27(9) 25.68 MgO 2.00 2.14 1.98 2.06 1.95 1.95 2.04 2.01 2.04 2.02(6) 3.20 FeO 0.28 0.30 0.28 0.23 0.54 0.28 0.25 0.22 0.22 0.29(9) - 3 Mn2O3 69.10 68.01 68.70 68.57 69.68 69.40 69.38 69.17 69.17 69.02(9) 69.70 4 Fe2O3 0.05 0.08 0.10 0.08 0.08 0.08 0.10 0.09 0.09 0.08(1) - Al2O3 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.02 0.01(1) - Σ 98.75 97.08 98.20 97.94 99.66 99.20 99.13 98.88 99.28 98.68(9) 98.58 Calculated on the basis of 4 oxygen atoms 2+ Mn 0.878 0.868 0.878 0.876 0.874 0.881 0.877 0.879 0.881 0.877(4) 0.820 Mg2+ 0.113 0.123 0.113 0.117 0.109 0.110 0.115 0.114 0.114 0.114(4) 0.180 Fe2+ 0.009 0.010 0.009 0.007 0.017 0.009 0.008 0.007 0.005 0.009(3) - ΣX 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 Mn3+ 1.998 1.997 1.997 1.998 1.997 1.998 1.997 1.997 1.997 1.997(0) 2.000 3+ Fe 0.002 0.002 0.003 0.002 0.002 0.002 0.003 0.003 0.003 0.003(0) - Al 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000(0) - ΣY 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 1 2+ 2+ 3+ ! represents the average of nine data points and gives an average formula of Mn 0.88Mg0.11Fe 0.01{Mn 2.00}O4 2A reported average of ten data points 3 MnO was converted to Mn2O3 by a factor of 1.1128. 4 FeO was converted to Fe2O3 by a factor of 1.1113. 2+ 3+ Ideal Hausmannite: 31.003 wt%. MnO and 68.997 wt%. Mn2O3, total wt.% = 100, Mn = 1 and Mn = 2 59

Figure 4.2. BSE image for hausmannite. The bright white regions are the euhedral hausmannite crystals, whereas the darker grey regions are henritermierite.

Table 4.2. Crystal structure refinement for hausmannite

Miscellaneous This study

Empirical formula Mn3O4 Formula weight 228.82 Temperature 293(2) K Wavelength 0.71073 Å

Crystal system Tetragonal

Space group I41/amd Unit-cell dimensions a = b = 5.7556(6) Å c = 9.4426(11) Å Volume 312.80(6) Å3 Z 4 Density (calculated) 4.859 Mg/m3

Absorption coefficient, µ 11.752 mm-1 3 Crystal size 0.05 x 0.05 x 0.06 mm Absorption, µR 0.2938 F (000) 428 θ range for data collection 4.15 to 60.63° Index ranges -14<=h<=14, -

14<=k<=14,

-23<=l<=23 Reflections collected 47254 Independent reflections 669 [R(int) = 0.0842] Completeness to θ = 60.63° 99.6 % Absorption correction None

Refinement method Full-matrix least-squares 2 on F Data / restraints / parameters 669 / 0 / 16 Goodness-of-fit on F2 1.144 Final R indices [I > 2σ (I)] R1 = 0.0213, wR2 = 0.0532

R indices (all data) R1 = 0.0277, wR2 = 0.0559

Extinction coefficient 0.003(1) -3 Largest diff. peak and hole 1.733 and -1.542 e.Å

Table 4.3. Atom coordinates and equivalent isotropic displacement parameters (Å2) for hausmannite. site s.o.f.s x y z U(eq)‡

Mn1(+3) 8d 0.975(7) 0 ½ ½ 0.007(1) Mn2(+2) 4a 0.976(7) ½ ¼ ⅝ 0.008(1) O1 16h 1.0 0.7776(1) ¼ 0.4914(1) 0.009(1) ‡ U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.

2 Table 4.4. Anisotropic† displacement parameters (Å ) for hausmannite.

U11 U22 U33 U23 U13 U12

Mn1 0.006(1) 0.006(1) 0.009(1) 0.001(1) 0 0 Mn2 0.008(1) 0.008(1) 0.008(1) 0 0 0 O1 0.007(1) 0.008(1) 0.011(1) 0 0.001(1) 0

† 2 2 2 The anisotropic displacement factor exponent takes the form: -2 π [ h (a*) U11 + ... + 2 h k a* b* U12]

Table 4.5. Selected distances (Å) and angles (°) for hausmannite.

This Study

†Mn1-O1 x4 1.9276(5) Mn1-O1 x2 2.2845(8) [6] 2.0466(9) Mn2-O1 x4 2.0361(8) †Mn1= Mn3+, which contains the Jahn-Teller effect.

Table 4.6. Bond-valence sums (v.u.) † for hausmannite.

Mn1 Mn2 Σ

O1 0.620 x4 0.501 x4 1.977‡ O1 0.236 x2 Σ 2.952 2.004 †Brown (2002) ‡1.977 = [(0.620 x 2) + 0.501 + 0.236]; Mn1 = Mn3+; Mn2 = Mn2+ 62

Table 4.7. Unit-cell parameters and bond distances for hausmannite (listed with increasing cell volume (V).

3 Composition a/Å c/Å c/a V/Å M – Os x 4 M – OL x 2 x 4 Reference

2+ 2+ 3+ 3+ † (Mn 0.94Fe 0.06)Σ1{Mn 0.88Fe 0.12}2O4 5.7779(4) 9.268(1) 1.6040 309.40 Baron et al. (1998) 2+ 2+ 3+ 3+ (Mn 0.74Fe 0.26)Σ1{Mn 0.96Fe 0.04}2O4 5.7482(5) 9.3752(12) 1.6310 309.77 1.9283(7) 2.2764(9) 2.0223(9) Bosi et al. (2010) 2+ 3+ 3+ † Mn {Mn 0.969Fe 0.031}2O4 5.7510(3) 9.3744(8) 1.6300 310.21 Baron et al. (1998) 2+ 3+ (Mn 0.74Zn0.26) Σ1Mn 2O4 5.7524(4) 9.4078(7) 1.6355 311.31 1.928(2) 2.282(2) 2.027(2) Bosi et al. (2002) 2+ 3+ (Mn 0.84Mg0.01Zn0.15) Σ1Mn 2O4 5.7535(7) 9.4282(15) 1.6387 312.10 1.928(2) 2.281(2) 2.034(2) Bosi et al. (2002) 2+ 3+ (Mn 0.83Mg0.03Zn0.14) Σ1Mn 2O4 5.7548(2) 9.4298(6) 1.6386 312.29 1.928(1) 2.284(2) 2.033(1) Bosi et al. (2002) 2+ 3+ † Mn Mn 2O4 5.7574(4) 9.4239(9) 1.6368 312.38 Baron et al. (1998) 2+ 3+ (Mn 0.83Mg0.03Zn0.15)Σ1Mn 2O4 5.7554(2) 9.4322(6) 1.6388 312.44 1.930(1) 2.281(2) 2.033(2) Bosi et al. (2002) 2+ 3+ (Mn 0.82Mg0.18)Σ1Mn 2O4 5.7550(3) 9.4365(8) 1.6397 312.54 1.9283(7) 2.2865(9) 2.032(1) Bosi et al. (2010) 2+ 2+ 3+ (Mn 0.88Mg0.11Fe 0.01)Σ1{Mn 1.00}2O4 5.7556(6) 9.4426(11) 1.6406 312.80(6) 1.9276(5) 2.2845(8) 2.0361(8) This study 2+ 3+ (Mn 0.89Mg0.02Zn0.10) Σ1Mn 2O4 5.7584(3) 9.4476(8) 1.6407 313.28 1.928(1) 2.287(2) 2.037(2) Bosi et al. (2002) 2+ 3+ (Mn 0.90Mg0.03Zn0.07)Σ1Mn 2O4 5.7591(4) 9.4464(11) 1.6403 313.31 1.927(1) 2.287(1) 2.038(1) Bosi et al. (2002) 2+ 3+ Mn Mn 2O4 5.762 9.439 1.6381 313.38 Aminoff (1926) 2+ 3+ Mn Mn 2O4 5.765(1) 9.442(2) 1.6378 313.81 1.930(1) 2.282(1) 2.040(1) Jarosch (1987) 2+ 3+ (Mn 0.98Zn0.02) Σ1Mn 2O4 5.7619(3) 9.4532(6) 1.6406 313.84 1.929(1) 2.285(1) 2.040(1) Bosi et al. (2002) 2+ 3+ (Mn 0.97Mg0.03Zn0.01) Σ1.01Mn 2O4 5.7607(5) 9.4601(12) 1.6422 313.94 1.928(1) 2.287(2) 2.041(2) Bosi et al. (2002) 2+ 3+ (Mn 0.98Zn0.02) Σ1Mn 2O4 5.7632(2) 8.4547(6) 1.6405 314.03 1.930(1) 2.286(1) 2.040(1) Bosi et al. (2002) 2+ 3+ (Mn 0.99Mg0.01)Σ1Mn 2O4 5.7625(3) 9.4611(7) 1.6418 314.17 1.929(1) 2.290(1) 2.040(1) Bosi et al. (2002) 2+ 3+ Mn Mn 2O4 5.7691(4) 9.4605(7) 1.6399 314.87 Satomi (1961) †For the nuclear structure, at T = 10 ° C 63

4.4 Discussion

Both EPMA and SCXRD results indicate that the sample contains one single phase.

Through EPMA data, heterogeneity was observed, which indicates minor variability in the chemistry of the sample. This heterogeneity results from the ability of the cation sites to host a variety of cations. In particular, the greatest heterogeneity is observed in the divalent cation site, where the cations may be Mn2+, Mg2+ or Fe2+. In this site, Mn2+ is the dominant cation, followed by Mg2+ and Fe2+. Significantly less variation is observed in the trivalent cation site, where the cations are Mn3+ or Fe3+, but the observed amount of Fe3+ is almost negligible at this site.

SCXRD results confirm tetragonal symmetry, with space group I41/amd, and unit-cell parameters a = b = 5.7556(6) Å and c = 9.4426(11) Å. The Mn2+ and Mn3+ cations occupy special positions, whereas the oxygen atoms occupy a general position (Table 4.3). The Mn1 –

O1 bond is the shortest, at 1.9276(5) Å x 4, where Mn1 is the Mn3+ cation; and contains the

Jahn-Teller effect.

The Jahn-Teller theorem states, “any nonlinear molecule in a degenerate ground state will undergo distortion that lowers its symmetry, thereby splitting the degenerate state” (Jahn and

Teller 1937, Senn 1992). The other Mn1 – O1 bond is the longest bond, at 2.2845(8) Å x 2, with the intermediate bond being the Mn2 – O1 bond, at 2.0361(8) Å x 4, where Mn2 is the Mn2+ cation.

The bond valence sums are reported in Table 4.6, which shows that Mn3+ has a valence sum of 2.95 rather than the expected 3.00, and Mn2+ has the expected valence sum of 2.00. The unit-cell parameters and bond distances for hausmannite are reported in Table 4.7, which shows a comparison between this study and previous studies of hausmannite. This study is the most similar to the work done in 2010 by Bosi and colleagues on end-member hausmannite, with Δa 64

and Δc = 0.0006, where Δn = n1-n2 and n1 and n2 represent data from this study and the study done by Bosi et al. (2010), respectively. Select structural variations for hausmannite are shown in Figure 4.3. Figure 4.3 displays the bond vs. (a) the a unit- cell parameter and (b) the c unit-cell parameter, as well as (c) the M – OL bond versus the c unit-cell parameter. Data from

Bosi et al. (2002) are represented by blue diamonds, and follow a linear trend line. The data from this study is represented by a red circle and falls near the linear trend-line.

5.766 Bosi et al. (2002) Bosi et al. (2010) Jarosch (1987) This Study 5.761

/Å 5.756 a

5.751 R2 = 0.8575 R² = 0.8575 5.746 9.480

9.458

Å 9.436 c / c 9.414

9.392 2 RR² = = 0.96867 0.9687 9.370 315.0 314.0

313.0 3 312.0 /Å V 311.0 310.0 R2 = 0.9479 R² = 0.94787 309.0 2.020 2.025 2.030 2.035 2.040 /Å Figure 4.3. Structural variations for hausmannite. Average distance varies linearly with (a) the a unit-cell parameter, (b) the c unit-cell parameter, and (c) the cell volume (Å3). The black linear trend line is related to data from Bosi et al. (2002).

4.5 Conclusion

The hausmannite in this study exists as one single phase with relatively homogeneous chemical composition. Hausmannite is tetragonal with space group I41/amd and forms a tetragonally distorted cubic spinel structure. The octahedral Mn3+ cation contains Jahn-Teller effects, where the octahedra are distorted.

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Appendix A

Back Scattered Electron Images

2 mm 2 mm

Figure A1. Back scattered electron (BSE) images for two crystals of henritermierite. The selected crystals are approximately 4mm in diameter, and did not show any inhomogeneous features.

Figure A2: Back scattered electron (BSE) images for (OH,F)-bearing spessartine. The selected crystal is approximately 4mm in diameter, and did not show any inhomogeneous features.

High-resolution Powder X-ray Diffraction (HRPXRD)

Figure A3: Complete HRPXRD trace for two-phase henritermierite, with space group I41/acd. The difference curve (Iobs – Icalc) is shown at the bottom. Short vertical lines indicate the allowed reflection positions. The inserts show the characteristic (200), (400) and (004) reflections.