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Chapter 20

Chapter 20. Simple compounds. Unusual mineral occurrences. Simple compounds. Rare (< 0.0002 wt% of crust), form under unusual conditions Minerals of: Native elements Solid solutions of native elements Intermetallic compounds Great economic interest

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Chapter 20

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  1. Chapter 20 Simple compounds. Unusual mineral occurrences

  2. Simple compounds • Rare (< 0.0002 wt% of crust), form under unusual conditions • Minerals of: • Native elements • Solid solutions of native elements • Intermetallic compounds • Great economic interest • Source for metals & elements; used for unique properties

  3. Crystal structure, morphology and physical properties • Morphology = external appearance • Morphology and physical properties influenced by crystal structure • Structure and bonding influences luster, density, conductivity, cleavage, melt point, hardness Octahedral Au Dendritic Ag

  4. Crystal structures, morphology and physical properties • Crystal structures • Native metals: simple – close-packed arrangement • Native sulfur: molecular structure – ring of 8 atoms with covalent bonds, stacked with Van der Waals bond between rings

  5. Crystal structures, morphology and physical properties • Native carbon: • Diamond: covalent bonds only with each atom surrounded by 4 atoms in tetrahedron • Graphite: hexagonal carbon sheets, covalent bonds within layer, Van der Waals bonds between layers • Fullerene: framework structure – trace mineral in some meteorites

  6. Native ElementsMetals • The Gold Group: • Gold • Called electrum if >10% Ag • Often associated with quartz and sulphides • Backing of currencies; jewellery; electronic industry • SA occurrence: Swaziland, Witwatersrand, Transvaal • Silver • Platelets, dendrites, wire like crystals in hydrothermal deposits • Rare mineral, sometimes in alloys with mercury: amalgam SA occurrence: Witwatersrand (with Au)

  7. Native ElementsMetals • The Gold Group • Copper: • Associated with copper minerals: cuprite, tenorite, malachite, chrysocolla • Native copper not very important economically • SA occurrence: Messina, Nababeep, Tsumeb, Phalaborwa • Lead • Vary rare as native lead

  8. Native ElementsMetals • The Platinum Group: • Pt, Ir, Os, Pd, Rh, Ru • Mostly occur as alloys of these PGEs • Ultramafic rocks • Native Pt commonly contains up to 30% Fe, Cu or Ni • Extremely rare • Jewellery; automobiles • SA occurrence: Wits; BIC: Merensky Reef, UG2, Platreef

  9. Native ElementsMetals • The Iron Group: • Two natural nativephases, occurs in meteorites: • Kamacite : native iron with up to 5-8% Ni • Taenite: native iron with up to 70% Ni • 2 Phases intergrownin meteorites • Rare in nature; major phases of commercial steel • Inclusions in diamonds

  10. Native ElementsMetals • The Mercury Group: • Native mercury: liquid mineral • Associated with cinnabar • Solid below -39°C – rhombohedral crystals • Silver amalgam: kongsbergite and landsbergite • Gold amalgam: 40% Au • Potarite: 35% Pd

  11. Bismuth Half-metals • The Bismuth Group • Bi: high T mineral in hydrothermal deposits and pegmatite veins; associated with molybdenite, wolframite, cassiterite, bismuthinite • Sb, As: low T metals; associated with Ag, realgar, orpiment, stibnite, galena, Co-sulphides, Ni-sulphides • SA occurrence: Namaqualand granite pegmatites Arsenic Antimony Arsenic

  12. Non-metals • The Tellurium Group • Te: associated with native gold, Au&Ag-tellurides, galena • Se: usually mixed with sulphur Selenium Tellurium

  13. Non-metals • The Sulphur Group • α-S; β-S; γ-S • α-S: most common and crystallize in orthorhombic system • Main economic deposits: • Volcanic activity (sublimation) • Micro-organism activity in sedimentary rocks • Use: manufacturing sulphuric acid fungicide

  14. Non-metals • Carbon • Diamond • Carbon atoms in tetrahedrons forming a face-centered isometric crystal structure • Covalent bonds in all directions – give high symmetry of structure and hardness • High meltpoint (3500°C); stable at very high pressures • Kimberlites and lamproites; placer deposits • Jewellery; ultra-hard material for cutting, drilling, abrasion • Graphite • Carbon atoms in corners of hexagonal sheets • Strong covalent bonds within sheets; weak Van der Waals bonds perpendicular to sheets • Very good cleavage along sheets • Lubricant

  15. Unusual conditions of formation • Metals such as Pt, Os, Ir, Au rare because inert – do not react easily • Needs low oxygen fugacity • Mantle • Pt in ultramafic rocks • Diamonds in kimberlite pipes & eclogites

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