A CRYSTALLOCHEMICAL STUDY OF GERSDORFFITE AND RELATED MINERALS A thesis presented in 1968 to The University of New South Wales for the Degree of Doctor of Philosophy This thesis has not been previously presented in whole or part to another University or Institution for a higher degree Peter Bayliss (ii) CONTENTS Page Summary 1. Introduction 3. Literature Survey: Crystal structure 5. Physical properties: 9 • Electrical and magnetic properties 9. Bonds and atomic radii 12. Order-disorder and forbidden reflections 13. Thermodynamic properties 14. Gersdorffite: Crystallography 15. Chemistry 16. Cobaltite: Crystallography 18. Chemistry 21. Ullmannite: Crystallography 22. Chemistry 22. Arsenopyrite: Crystallography 24. Chemistry 26. Glaucodot: Crystallography 27. Chemistry 28. Gudmundite: Crystallography 29. Chemistry 30. Literature review 31. (iii) Page Methods: Synthesis 33. X-ray diffraction of synthetic powder 37. Natural powder examination 42. Single crystal X-ray diffraction 43. Crystal structure computations 47. Results: Gersdorffite: Natural material 49. Crystal structure Slovakia Pa3 54. Wolfsberg P213 55. Leichtenberg PI 58. Synthesis 62. Cobaltite: Natural material 64. Crystal stxucture 66. Synthesis 67. Ullmannite: Natural material 69. Crystal structure 70. Synthesis 72. Arsenopyrite, glaucodot and gudmundite: Natural material 73. Synthesis 74. Pyrite, cattierite and vaesite 74. (iv) Page Discussion: Methods 76. Natural material 77. Synthesis 78. Bernd angles and distances 80. Crystal structure accuracy 83. Crystal structure 85. Crystal structure relations 87. Acknowledgments 94. References 95. Data appendix: Unit cell data 107. X-ray diffraction powder intensities, optical anisotropy, zoning and paragenesis 114. Crystallographic data 118. Chemical data 133. Heating experiments 135. Publication List 137. (v) LIST OF TABLES Page 1. Unit Cell Data la. Gersdorffite: literature and natural 107. lb. artificial 108. le. Cobaltite: literature and natural 109. Id. artificial 110. le. Ullmannite: literature and natural 111. If. Ullmannite and gersdorffite: artificial 112. lg. Arsenopyrite, glaucodot and gudmundite 113. 2. X-ray Diffraction Powder Intensities, Optical anisotropy, Zoning and Paragenesis. 2a. Gersdorffite 114. 2b. Cobaltite 115. 2c. Ullmannite 116. 2d. Arsenopyrite, glaucodot and gudmundite 117. 3. Crystallographic Data: Observed reflection amplitudes, Calculated structure factors, Atomic and Thermal parameters, lnteratomic distances, Ato'!lic radii, and Tetrahedral and Octahedral a ogles. 3a. Gersdorffite: Slovakia, Pa3 118. (vi) Page 3b. Wolfsberg, P213 120. 3c. Leichtenberg, PI 122. 3d. Gersdorffite: synthetic arsenic-rich 124. 3e. synthetic 125. 3f. synthetic sulphur-rich 126. 3g. Cobaltite: Pa3, Giese and Kerr (1965) 127. 3h. Pca21, Giese and Kerr (1965) 128. 3j. Ullmannite: Takeuchi (1957) 129. 3k. synthetic 131. 31. Pyrite, cattierite and vaesite 132. 4. Olemical Data. 4a. hnpurities (ppm) in specpure materials 133. 4b. Spectrographic analyses 134. 5. Heating Experiments. Sa. Gersdorffite 135. Sb. Cobaltite 136. (vii) LIST OF FIGURES Page I . Pyrite and marcasite crystal structures 7. 2. FeAsS-CoAsS-NiAsS ternary diagram of Klemm (1965) 16. 3. Cell edge versus gersdorffite composition I 7. 4. Cell edge versus cobaltite composition 67. 5. Cell edge versus ullmannite composition 71. 6. NiSb2-NiAs2-NiS2 ternary diagram 72. 7. Diagrammatic sketch of FeAsS-CoAsS join 74. 8. Positional displacements in crystal structures of gersdorffite 85. 9. Diagrammatic sketch of CoAsS-NiAsS join 92. I. SUMMARY Data are presented from natural samples of gersdorffite and related minerals for unit cell size, powder X-ray diffraction reflection intensities, optical anisotropism, zoning, and paragenesis. Three single crystal structure analyses of distinct gersdorffite crystals, and single crystal structure refinements of cobaltite and ullmannite elucidate the non •metal atom ordering and structure distortion. For gersdorffite and related minerals, the absence of both the 001 and 011 reflections in their powder patterns indicate a disordered cubic structure Pa3; the absence of the 001 reflection and the presence of the 011 reflection in their powder patterns indicate a partially ordered cubic structure P213; the presence of both the 001 and OU reflections in their powder patterns indicate a non-cubic distorted structure Pca21 or Pl. For gersdorffite and related minerals, the amount of structure distortion is related to the 001 reflection intensity in their powder patterns and also to their optical anisotropism strength. These are semi­ quantitatively related to the thermal stability of the distorted structure. This thermal stability increases with the compositional substitution of cobalt for nickel, which gives a decrease in the unit cell size. An order•disorder change occurs before a distortion release in gersdorffite with a large unit cell size, whereas only a distortion release is observed in cobaltite and gersdorffite with a small unit cell size. 2. Synthetic gersdorffite results show that ordering of the non •metal atoms decreases with a rise in formation temperature and deviation from the stoichiometric composition. The small distortion (small 001 reflection intensity in a powder pattern) in synthetic cobaltite is attributed to slow reaction rates and a possible hysteresis cycle. The prepared synthetic cobaltite compositions range from CoAs0 • 86s1. l4 to CoAs0 _42S1. 58 at 550° C. The prepared synthetic ullmannite is stoichiometric NiSbS, al• though arsenic may substitute extensively for antimony and sulphur. Syn­ thetic materials with stoichiometric compositions and limited metal atom substitution were produced by the small evacuated tube method in contrast to the LiCl•KCl melt method. 3. INTRODUCTION In recent years, extensive research work has been conducted into sulphides. This research work includes phase equilibrium studies such as the geologically significant sulphide-type systems, which were reviewed and evaluated by Kullerud (I 964). The limits of solid solution in these phase equilibrium studies indicate the compositional range of each mineral. Other lines of research work include crystallographic determinations such as the unit cell data tanilated by Donnay and Donnay (1963), the crystal structures described by Bragg and Claringrull (1965), and the X-ray dif• fraction powder data collated by Berry and Thompson (1962). Other lines of research involve the geological applicability of sulphides, such as the review by Kullerud (1959) on the use of sulphide-type systems as geological thermometers. The extent of knowledge about gersdorffite and its related minerals is reviewed in the following literature survey under sections on crystal structure, physical properties, and the crystallography and chemistry of each mineral. The aim of this investigation was to obtain data to fill omissions and check inconsistencies in the published data, which are summarized in the literature review. The methods used to investigate these problems were mainly mineral syntheses from pure elements, polished section 4. studies, and X-ray diffraction powder and single crystal structure analyses. The data obtained are presented in the results section. These data are then combined with the published data to present a comprehensive theory to account for all the data in the discussion. A major difficulty in the experimental work was to decide when the synthetic minerals had reached equilibrium. After the start of this project in 1964, additional valuable information was added to the literature by Giese and Kerr (1965) on the crystal structure of cobaltite and by Klemm (1965) on the chemical composition of (Fe, Co, Ni)AsS compounds. 5. LITERATIJRE SURVEY CRYSTAL STRUCTURE The crystal structures of AX2 compounds to which gersdorffite and its related minerals belong are grouped into several major types by Wycoff (1963) of symmetrical (predominantly ionic), layer (moderately ionic), molecular (covalent), and metallic. In a symmetrical structure, A and X differ widely in electro-negativity to produce mainly ionic bonding so the structure type is determined by their ionic radii (r). If r1/r~,,. O. 7 then the resultant 8:4 co-ordination is called the fluorite-type structure; if O. 7 > rA /r)( "" 0.3 then the resultant 6:3 co-ordination is called the rutile or cassiterite-type structure; and if rl/r~ < 0.3 then the resultant 4:2 co-ordination is called the silica-type structure. In a layer structure, a resonance between predominantly covalent and ionic bonding occurs within each layer, whereas the weak bond of Van der Waals holds the layers to• gether. The types of layer structure include cubic close packing for example CdCl2, hexagonal close packing for example CdI2, hexagonal packing for example MoS2, and approximate close packing for example AlO(OH). The molecular structure varieties are the infinite such as FeS2 and the discrete such as co2 with covalent bonding within molecules and Van der Waals forces between molecules. A metallic structure, for example Al82, has intermetallic bonding. Sulphides, arsenides and related AX 2 compounds of elements in the 8 sub-group of the periodic table (sulphide-chemistry) differ from the 6. corresponding oxide compounds because firstly sulphur, selenium, telurium, arsenic, antimony and bismuth atoms are larger and more easily polarized than oxygen atoms; secondly these non-metal atoms can form covalent bonds between each other; and thirdly arsenic, antimony and bismuth have semi-metallic properties. There are