EARTH MATERIALS IVEARTH MATERIALS IV

Rock-forming minerals: non-silicatesRock-forming minerals: non-silicates

Professor Peter DoyleP.doyle@imperial.ac.uk

Profdoyle@btinternet.com

CLASSIFICATION OF MINERALSCLASSIFICATION OF MINERALS

Mineral class Example

SILICATES Olivine (Mg,Fe)2 SiO4 CARBONATES Calcite CaCO3 SULPHATES Anhydrite CaSO4

OXIDES Hematite Fe2O3

SULPHIDES Pyrite FeS2

HYDROXIDES Brucite Mg(OH)2

HALIDES Halite NaClNATIVE Gold Au

Carbonates• Calcite CaCOCalcite CaCO33 – – trigonaltrigonal

• Aragonite CaCOAragonite CaCO33 – – orthorhombicorthorhombic

• Calcite & Aragonite are polymorphs - same composition, different crystal structure

• Dolomite CaMg (CODolomite CaMg (CO33))22 – trigonal – trigonal

Calcite

Calcite & Aragonite

stable at different

temperatues

CARBONATE MINERALS

CALCITE CaCO3 DOLOMITE CaMg(CO3)2

Ca Ca

Mg

Ca

Mg

Ca

Ca

Carbonates: most commonly found in carbonate most commonly found in carbonate sediments e.g. limestonessediments e.g. limestones

Modern carbonate sedimentsmay 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 CaSO4.2HGypsum CaSO4.2H22O– O– monoclinicmonoclinic

• Anhydrite CaSOAnhydrite CaSO44 – – orthorhombicorthorhombic

• 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 = 3.5• 3 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

HalidesHalite NaClHalite NaCl• Chloride minerals

characterised by salty taste

• Readily soluble in water• H<3

Fluorite CaFFluorite CaF22

• Well defined cleavage • vitreous lustre • H = 4 • Variable colours - white

streak

HALITE

FLUORITE

Oxides

Hematite FeHematite Fe22OO33 – cubic – cubicMagnetite FeMagnetite Fe22OO44 – cubic – cubic

• Most oxides opaque with metallic lustre• Some transparent with vitreous-resinous lustre• H >5

Botryoidal hematite

Streak - constant

Sulphides

Pyrite FeSPyrite FeS22 – cubic – cubicGalena PbS – cubicGalena PbS – cubicChalcopyrite Cu FeSChalcopyrite Cu FeS22 – cubic – cubic Sphalerite ZnS – cubicSphalerite ZnS – cubic

• Sulphides opaque • metallic lustre• H ≤6

PYRITE

GALENA