EARTH MATERIALS IV Rock-forming minerals: non-silicates Professor Peter Doyle

CLASSIFICATION OF MINERALS Mineral classExample SILICATES Olivine (Mg,Fe) 2 SiO 4 CARBONATES Calcite CaCO 3 SULPHATES Anhydrite CaSO 4 OXIDES Hematite Fe 2 O 3 SULPHIDES Pyrite FeS 2 HYDROXIDES Brucite Mg(OH) 2 HALIDES Halite NaCl NATIVE Gold Au

Carbonates Calcite CaCO 3 – trigonalCalcite CaCO 3 – trigonal Aragonite CaCO 3 – orthorhombicAragonite CaCO 3 – orthorhombic Calcite & Aragonite are polymorphs - same composition, different crystal structure Dolomite CaMg (CO 3 ) 2 – trigonalDolomite CaMg (CO 3 ) 2 – trigonal Calcite Calcite & Aragonite stable at different temperatues

CARBONATE MINERALS CALCITE CaCO 3 DOLOMITE CaMg(CO 3 ) 2 Ca Mg Ca Mg Ca

most commonly found in carbonate sediments e.g. limestones Carbonates: most commonly found in carbonate sediments e.g. limestones Modern carbonate sediments may contain aragonite & calcite Ancient carbonates rarely contain aragonite which transforms on burial to calcite

Carbonates in hand specimen Vitreous to pearly lustre Hardness, generally <5 Trigonal carbonates show perfect rhombic cleavage Calcite and aragonite effervesce, readily soluble in cold dilute HCl Dolomite only reacts in hot dilute HCl

Sulphates Gypsum CaSO4.2H 2 O– monoclinicGypsum CaSO4.2H 2 O– monoclinic Anhydrite CaSO 4 – orthorhombicAnhydrite CaSO 4 – orthorhombic Vitreous to pearly lustre Hardness <4 Usually occur in evaporite depositsUsually occur in evaporite deposits Gypsum twin Twin plane

EVAPORITES IN PLAYA LAKE EVAPORITES IN MARINE SALINA

Gypsum in hand specimen Tabular, fibrous, granular,massive (alabaster) habits Often twinned Soft (H = 2) One perfect + 2 good cleavages Gypsum in evaporite deposits

Anhydrite in hand specimen H = Good/perfect intersecting cleavages at right angles Fibrous, granular habits Nodular anhydrite after gypsum in modern arid tidal flat, ( sabkha ) Abu Dhabi, Arabian Gulf Nodular ‘chicken wire’ anhydrite in Permian sediments, UK

Halides Halite NaCl Chloride minerals characterised by salty taste Readily soluble in water H<3 Fluorite CaF 2 Well defined cleavage vitreous lustre H = 4 Variable colours - white streak HALITE FLUORITE

Oxides Hematite Fe 2 O 3 – cubic Magnetite Fe 2 O 4 – cubic Most oxides opaque with metallic lustre Some transparent with vitreous-resinous lustre H >5 Botryoidal hematite Streak - constant

Sulphides Pyrite FeS 2 – cubic Galena PbS – cubic Chalcopyrite Cu FeS 2 – cubic Sphalerite ZnS – cubic Sulphides opaque metallic lustre H ≤6 PYRITE GALENA