Simple compounds. Unusual mineral occurrences

Minerals of: – Native elements – Solid solutions of native elements – Intermetallic compounds Rare (< wt% of crust), form under unusual conditions Great economic interest – Principal source for many metals & elements – Gold, platinum, osmium, iridium, silver, sulfur – Used for unique properties of mineral – Diamond, graphite

 Morphology and physical properties influenced by crystal structure and types of bonds  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; linking of 5- and 6- membered rings

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: rare, 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

The Bismuth Group – Bi: high T metal in hydrothermal deposits and pegmatite veins; associated with molybdenite, wolframite, cassiterite, bismuthinite – Sb, As: low T metal; associated with Ag, realgar, orpiment, stibnite, galena, Co-sulphides, Ni-sulphides – SA occurrence: Namaqualand granite pegmatites Bismuth AntimonyArsenic

 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