Chapter 20

Chapter 20Simple compounds.Unusual mineral occurrencesSimple compoundsRare (< 0.0002 wt% of crust), form under unusual conditionsMinerals of:Native elementsSolid solutions of native elementsIntermetallic compoundsGreat economic interestSource for metals & elements; used for unique propertiesCrystal structure, morphology and physical propertiesMorphology = external appearanceMorphology and physical properties influenced by crystal structureStructure and bonding influences luster, density, conductivity, cleavage, melt point, hardness

Octahedral AuDendritic AgCrystal structures, morphology and physical propertiesCrystal structuresNative metals: simple close-packed arrangementNative sulfur: molecular structure ring of 8 atoms with covalent bonds, stacked with Van der Waals bond between rings

Crystal structures, morphology and physical propertiesNative 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

Native ElementsMetalsThe Gold Group:GoldCalled electrum if >10% AgOften associated with quartz and sulphidesBacking of currencies; jewellery; electronic industrySA occurrence: Swaziland, Witwatersrand, TransvaalSilverPlatelets, dendrites, wire like crystals in hydrothermal depositsRare mineral, sometimes in alloys withmercury: amalgamSA occurrence: Witwatersrand (with Au)

Native ElementsMetalsThe Gold GroupCopper:Associated with copper minerals: cuprite, tenorite, malachite, chrysocollaNative copper not very important economicallySA occurrence: Messina, Nababeep, Tsumeb, PhalaborwaLeadVary rare as native lead

Native ElementsMetalsThe Platinum Group:Pt, Ir, Os, Pd, Rh, RuMostly occur as alloys of these PGEsUltramafic rocksNative Pt commonly contains up to 30% Fe, Cu or NiExtremely rareJewellery; automobilesSA occurrence: Wits; BIC: Merensky Reef, UG2, Platreef

Native ElementsMetalsThe Iron Group:Two natural native phases, occurs in meteorites:Kamacite : native iron with up to 5-8% NiTaenite: native iron with up to 70% Ni2 Phases intergrown in meteoritesRare in nature; major phases of commercial steelInclusions in diamonds

Native ElementsMetalsThe Mercury Group:Native mercury: liquid mineralAssociated with cinnabarSolid below -39C rhombohedral crystalsSilver amalgam: kongsbergite and landsbergiteGold amalgam: 40% AuPotarite: 35% Pd

Half-metalsThe Bismuth GroupBi: high T mineral in hydrothermal deposits and pegmatite veins; associated with molybdenite, wolframite, cassiterite, bismuthiniteSb, As: low T metals; associated with Ag, realgar, orpiment, stibnite, galena, Co-sulphides, Ni-sulphidesSA occurrence: Namaqualand granite pegmatites

BismuthAntimonyArsenicArsenicNon-metalsThe Tellurium GroupTe: associated with native gold, Au&Ag-tellurides, galenaSe: usually mixed with sulphur

TelluriumSeleniumNon-metalsThe Sulphur Group-S; -S; -S-S: most common and crystallize inorthorhombic systemMain economic deposits: Volcanic activity (sublimation)Micro-organism activity in sedimentary rocksUse: manufacturing sulphuric acidfungicide

Non-metalsCarbonDiamondCarbon atoms in tetrahedrons forming a face-centered isometric crystal structureCovalent bonds in all directions give high symmetry of structure and hardnessHigh meltpoint (3500C); stable at very high pressuresKimberlites and lamproites; placer depositsJewellery; ultra-hard material for cutting, drilling, abrasionGraphiteCarbon atoms in corners of hexagonal sheetsStrong covalent bonds within sheets; weak Van der Waals bonds perpendicular to sheetsVery good cleavage along sheetsLubricant

Unusual conditions of formationMetals such as Pt, Os, Ir, Au rare because inert do not react easilyNeeds low oxygen fugacityMantlePt in ultramafic rocksDiamonds in kimberlite pipes & eclogites