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By RATEB M. ABU-EID B.Sc ABSORPTION SPECTRA ANTD PHASE TRANSFORMATIONS OF MINERALS AT PRESSURES UP TO 200 KILOBARS by RATEB M. ABU-EID B.Sc.(Hons), Alexandria University 1968 SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY January, 1975 Signature of Author Department of Earth and Planetary Sciences, January 27, 1975 Certified by Thesis Supervisor Accepted by c1 0I -2- ABSTRACT HIGH PRESSURE ABSORPTION SPECTRA AND PHASE TRANSFORMATIONS OF MINERALS AT PRESSURES UP TO 200 KILOBARS by Rateb M. Abu-Eid Submitted to the Department of Earth and Planetary Sciences on January 27, 1975 in partial fulfillment of the requirements for the Degree of Doctor of Philosophy The absorption spectra of minerals containing a variety of transition metal ions were investigated at pressures ranging from 1 atm. to 200 kb. using the Cary 17 spectrophotometer with a newly designed optical microscope attachment and the diamond anvil pressure cell. The general results indicate that with increasing pressure, spin-allowed crystal field bands shift to higher energies, whereas charge transfer bands shift to lower energies; spin-forbidden bands, on the other hand, did not show any significant shift. The rate of the energy shift (cm-l/kb.) varies from one band to the other depending on the nature of the transition metal ion and the coordination site symmetry. From the high pressure spectra, energy level diagrams were constructed at elevated pressures. Such diagrams provided the energy values of the crystal field splitting, 1ODq, and the crystal field stabilization energy, CFSE, parameters at various pressures. The site compressibilities of Fe2 +, Mn 3 +, and Cr 3+ contained in orth- oferrosilite, piemontite, and uvarovite were estimated at various pressures and correlated with the compressibilities of the bulk mineral phases dontaining them. It appears that the Si04 tetra- hedra are the least compressible sites, whereas the 8-fold coord- ination sites (Ca site) are the most compressible ones. The degree of covalency of the metal-ligand bond was esti- mated in uvarovite and Fe3+-bearing minerals. There is a very slight decrease in Racah parameter B with increasing pressure (6-10 cm-l/100 kb.), indicating that the ionic character of the M-L bond may not be changed significantly under pressures of up to 200 kb. The d-energy levels constructed at high pressures provided information on the degree of regularity or distortion of the transition metal ion site. In most cases, it appears that distorted sites tend to be more regular at elevated pressures, -3- and the longer M-0 bond is shortened substantially more than the shorter one as pressure increases. No evidence has been obtained for any pressure-induced spin-pairing in Fe 2+ in gillespite mineral or in any other minerals. Nevertheless, from the rates of band energy shifts with pressure, it appears that spin-pairing may take place in the lower mantle in the FeO phase. Pressure-induced reduction of Cr6+, V5+, Mn 7+, and Cu 2+ ions contained in crocoite, vanadinite, KMnO 4 , and Egyptian- blue, occurs at pressures ranging from 50-120 kb.. On the other hand, no reduction was detected in the other cations such as Fe3+, Cr3 +, and Mn 3 + with increasing pressure up to 200 kb.. The high pressure spectra were used to distinguish crystal field bands from charge transfer bands, (e.g. Ti3 + in lunar pyroxene), and to give appropriate assignments to many of the observed absorption features in the spectra of minerals. In the course of this study, many high pressure polymorphs were identified from the high pressure spectra. Some of those were studied intensively using other techniques, such as x-ray and Mbssbauer, and others remained to be investiaated further. From the high pressure spectra, the minerals gillespite, azur- ite, crocoite, and vanadinite are believed to transform to other structures. In all of these minerals, discontinuous changes in their colors and spectra were observed at various pressures. High pressure x-ray studies are warranted to identify the crystal structures of the new polymorphs. THESIS SUPERVISOR: Roger G. Burns TITLE: Professor of Mineralogy and Geochemistry - 4 TABLE OF CONTENTS TITLE PAGE....................... ABSTRACT. .. .. .. .. .. .. .. TABLE OF CONTENTS .. .. .. .. .. .. .. .. .. 4 INDEX OF FIGURES IN THE TEXT.. ... .. .. .. ... .. 7 INDEX OF TABLES IN THE TEXT - ..- . .. .. .. 13 ACKNOWLEDGMENTS . .. .. ..... ... .. .. 14 Chapter I: INTRODUCTION. ..... ...... .... 15 Chapter II: THEORETICAL BASIS . .. .. .. .. .. 19 II-1. Introduction. ... ...... .. 19 11-2. Crystal Field Theory. ...... 20 11-3. Ligand Field Theory and Racah Parameters. .... ... .. 24 11-4. Molecular Orbital Theory and Charge Transfer Spectra. ... ... 31 11-5. Effect of Pressure On Crystal Field and Charge Transfer Bands. .. 36 Chapter III: APPARATUS AND EXPERIMENTAL METHODS.. .. .. 44 III-1. Spectral Apparatus.. ... .. .. 44 111-2. High Pressure Instruments. .. .. 54 111-3. Sample Preparation For Spectral Studies.. .......... ... 62 111-4. Pressure Calibration ... ... 72 111-5. Experimental Difficulties.. ... .. 78 Chapter IV: EFFECT OF PRESSURE ON CRYSTAL FIELD BANDS ... ... .... ...... 86 IV-l. Introduction. .. ... .. ... .. 86 IV-2. Pyroxenes ... ... ... ... .. 87 IV-2.1 Orthoferrosilite.. .. .... 91 IV-2.2 Bronzite .. .. ... ill IV-2.3 Lunar Ti-augite.. ... .. 116 IV-2.4 Blue Omphacite .. ... 131 IV-2.5 Rhodonite. .. .. .. 141 IV-3. Olivine .. .. &. ... .. 148 IV-4. Garnets .. .... ..... ... 165 IV-4.1 Almandine. ... .... 168 IV-4.2 Andradite. .. ... .... 182 IV-4.3 Uvarovite. .... ..... 189 - 5 - page IV-5. Epidotes. _+.. .... ...... 193 IV-5.1 Fe 3+-Epidotes. .... 196 IV-5.2 Mn -Epidotes.. .... 201 IV-6. Miscellaneous Minerals.. .... 214 IV-6.i Staurolite.. ...... 214 IV-6.2 Lusakite.. ....... 220 IV-6.3 Melilite.. ....... 230 IV-6.4 Glaucophane.. ... ... 246 IV-6.5 Dioptase.. .... ... 255 IV-6.6 Gillespite.. .. .. .. 263 Chapter V: EFFECT OF PRESSURE ON CHARGE TRANSFER BANDS.. ............ 264 V-l. Introduction.. ........ 264 V-2. Band Energies, Intensities and Influence of Pressure .. .. 265 V-3. Effect of Pressure On M-M Charge Transfer Bands.. .. ... 268 V-3.1 Glaucophane.. ... ... 269 V-3.2 Blue-Omphacite.. .... 276 V-3.3 Vivianite ...... 281 V-3.4 Fayalite and Orthoferrosilite. .. 290 V-4. Effect Of Pressure On M-L and L-M Charge Transfer Absorption.. .. .... .... 291 V-4.1 Crocoite.. .... ... 293 V-4.2 Vanadinite.. ...... 307 V-4.3 KMnO .* . ...... 315 V-4.4 Hematite .. ...... 324 V-4.5 Silicates ...... 330 Chapter VI: PRESSURE INDUCED PHASE TRANSFORMATIONS IN: GILLESPITE, EGYPTIAN BLUE AND AZURITE ... ........... 333 VI-1. Introduction.. ... ...... 333 VI-2. Gillespite.. .......... 335 VI-2.1 Introduction.. ...... 335 VI-2.2 Experimental Methods. .. 336 VI-2.3 Results.. ..... ... 337 VI-2.4 Discussion.. .. ..... 363 VI-3. Egyptian Blue ......... 377 VI-4. Azurite ..... ....... 384 Chapter VII: DISCUSSION .... .... ... ... 396 VII-l. Energy Shifts Of Spectral Bands With Pressure ...... ... 396 VII-2. The Degree of Covalency and Racah Parameter B at Elevated Pressures 400 - 6 - page VII-3. Site Comnressibilities From 1ODq Values. .. ... 408 VII-4. Crystal Field Stabilization Energies (CFSE).. ....... 412 VII-5. Pressure Induced Reduction. .. 421 VII-6. Pressure induced Spin- Pairing ...... ....... 428 VII-7. The Trend Of Pressure Induced Changes In The Structure Of Silicate Minerals . ......... .. 437 VII-8. Absorption Spectra And Radia- tive Heat Transfer . o.. ..... 443 References. ....... .. .. .. 46 Vita ........ 4747 - 7 - INDEX OF FIGURES IN THE TEXT (Page numbers refer to figure captions, the figures will be found on the page immediately following.) FIGURE PAGE II-1. Octahedral array of ligands and position of electron at point P described in spherical coordinates . .. .. 21 11-2. Energy levels of the free cation, free ligands, and molecular unit (ML6 ) - - - - *. 33 11-3. Schematic representation of conduction and valence bands....--.-.-.- .. .. 38 11-4. Potential energy wells of ground and excited states . .. .. .. .. .. .. .. 41 III-1. Photograph showing Cary 17 spectrophoto- meter *.... .. .. .. .. .. 45 111-2. Photograph showing reference and sample microscopes .. .. .. .. .. .. .. .. 47 111-3. Reflecting focusing system . -. -. 50 111-4. Spectrophotometer optical attachment . 52 111-5. Light path in the new optical system . 55 111-6. Photograph of UP style pressure cell mounted on universal x-ray table . - - - 58 111-7. Photograph of UXP type pressure cell mounted inside Debye-Scherrer powder x-ray camera . -. - - - - - - -. 60 111-8. Photograph of PP type diamond cell mounted on eucentric goniometer head . .. 63 111-9. Gasketing method . - - - - - - - - - -. 66 III-10. Photograph of silver iodide (AgI) powder between two diamond anvils ...... ...... 71 III-11. Geometries of x-ray reflections from NaCl between the anvil faces .. .. ...... 76 111-12. Visible spectra of a pair of diamond anvils.used in the pressure cell .. ...... 79 III-13a. Pressure distribution in ungasketed sample of powdered NaCl in diamond anvil cell ..... .'. ......... 83 III-13b. Pressure contours for sample of nickel dimethylglyoxime in NaCl (1:3) in diamond-anvil cell . 83 IV-l. The crystal structure of diopside on z--axis . - - - - - - - - -- - - - -.. -. -. - 88 IV-2. Views of Ml and M2 sites in orthoferro- silite . ....... .. .. .. .* *. 92 IV-3. Crystal structure of orthoferrosilite . ... 94 IV-4. a-spectra of orthoferrosilite at 1 atm. and 20 kb. .
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