Transcript Chapter 20
Simple compounds. Unusual mineral occurrences • • 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 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 • 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 – – 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 • 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) • – 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 • 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 • – The Iron Group: Two natural native phases, occurs in meteorites: • Kamacite : native iron with up to 5-8% Ni • Taenite: native iron with up to 70% Ni – 2 Phases intergrown in meteorites – Rare in nature; major phases of commercial steel – Inclusions in diamonds • 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 – Bismuth • 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 Antimony Arsenic Arsenic The Tellurium Group Te: associated with native gold, Au&Ag-tellurides, galena Se: usually mixed with sulphur Tellurium Selenium • 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 • 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 • • 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