Simple compounds. Unusual mineral occurrences. Minerals of: – Native elements – Solid solutions of native elements – Intermetallic compounds Rare (< 0.0002.

CHAPTER 20Simple compounds.

Unusual mineral occurrences

Simple compounds

• Minerals of:– Native elements– Solid solutions of native elements– Intermetallic compounds

• Rare (< 0.0002 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

Crystal structure, morphology and physical properties

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, 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

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; linking of 5- and 6-membered rings

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: amalgamSA occurrence: rare,

Witwatersrand (with Au)

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

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

Native ElementsMetals

• 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

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

Half-metals

• 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

Antimony ArsenicArsenic

Non-metals The Tellurium Group

Te: associated with native gold, Au&Ag-tellurides, galena

Se: usually mixed with sulphur

TelluriumSelenium

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 acidfungicide

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

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