EARTH MATERIALS III Rock-forming minerals: 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

Silicates are dominant minerals

Silicate structure Silica tetrahedraBased on Silica tetrahedra four oxygen ions silicon ionSilica tetrahedra formed by four oxygen ions surrounding & sharing electrons with silicon ion Silicate structure based on repetition of tetrahedra

STRUCTURE of SILICATE MINERALS Important examples Olivine, Garnet Pyroxenes Amphiboles Micas Quartz

orthosilicatesOlivines are orthosilicates Forsterite Mg 2 SiO 4 – Fayalite Fe 2 SiO 4 Isomorphous SeriesForms an Isomorphous Series Isomorphs – different chemical compositions but same crystal structure OLIVINES (Mg,Fe) 2 SiO 4 No oxygen sharing - tetrahedra bonded to ion (Mg, Fe) between them

OLIVINES IN HAND SPECIMEN Granular, pale to dark green crystals No cleavage apparent Hardness of 6.5

GARNETS (Ca,Mg,Fe 2 +,Mn) 3 (Al,Fe 3+,Cr) 2 Si 3 O 12 Also orthosilicatesAlso orthosilicates Cubic symmetry Several named varieties, e.g. Pyrope (Mg 3 Al 2 Si 3 O 12 ) Almandine (Fe 3 Al 2 Si 3 O 12 ) Almandine

GARNETS IN HAND SPECIMEN dark equidimensional crystals no cleavage hardness = 6-7.5

PYROXENES Single chain silicatesSingle chain silicates orthopyroxenes) clinopyroxenes)Can be orthorhombic (orthopyroxenes) or monoclinic (clinopyroxenes) Many named varieties, e.g.: Augite – Ca clinopyroxene Aegirine – Na clinopyroxene Enstatite – orthopyroxene Silica tetrahedra linked in chain by shared oxygens, linked laterally by various cations, notably Mg, Fe, Al, Ca, Na

PYROXENES IN HAND SPECIMEN Dark coloured but varies depending on Fe/Mg Elongate prismatic habits Intersecting cleavages at 87/93 Hardness Cleavages in section

Aegirine crystals

AMPHIBOLES Double chain silicatesDouble chain silicates Orthorhombic (orthoamphiboles) or monoclinic (clinoamphiboles) Many named varieties, e.g. –Hornblende Ca clinoamphibole –Glaucophane Na clinoamphibole –Anthophyllite orthoamphibole Parallel chains joined by shared oxygens, linked laterally by various cations, notably Mg, Fe, Al, Ca, Na

AMPHIBOLES IN HAND SPECIMEN Fe/Mg varieties dark coloured; Al rich varieties light coloured Crystals typically acicular, even fibrous (but not always) Two cleavages intersect at 124/56 degrees Difficult to differentiate between pyroxenes, except for cleavages Hardness 5–6 Amphibole Pyroxene

HORNBLENDE

ACTINOLITE

GLAUCOPHANE

MICAS Sheet silicatesSheet silicates Monoclinic Most common examples: –Muscovite KAl 2 (Si 3 AlO 10 )(OH) 2 –Biotite K(Mg,Fe) 2 (Si 3 AlO 10 )(OH) 2 Each tetrahedron linked to three others by shared oxygens, sheets bonded by cations, notably K, Mg, Fe, Al

MICAS IN HAND SPECIMEN Pearly to vitreous lustre Colour varies –Muscovite -- –Muscovite -- usually colourless –Biotite -- –Biotite -- dark brown, colour increases with increasing Fe Platy, plates flexible with one perfect cleavage Hardness 2-3 MuscoviteBiotite

CHLORITE Sheet silicatesSheet silicates with different crystal structure to micas (Mg,AL,Fe) 6 (Si,Al) 4 O 10 (OH) 8 Monoclinic Typically green, colour increases with increasing Fe Platy, with one perfect cleavage Hardness 2-3 CHLORITE

QUARTZ & CHALCEDONY Framework silicatesFramework silicates QUARTZ (SiO 2 ) most common of the silica minerals CHALCEDONY microcrystalline quartz with sub- microscopic pore spaces filled with water Each tetrahedron shares all its oxygens with other silica in Quartz (SiO 2 )

QUARTZ IN HAND SPECIMEN Colourless, but many coloured varieties due to chemical impurities (e.g. amethyst) Vitreous lustre No cleavage – conchoidal fracture Hardness 7

CHALCEDONY IN HAND SPECIMEN AgateAgate - banded variety FlintFlint - dark nodular variety ChertChert - rock composed of chalcedony Massive with conchoidal fracture Hardness 7

Also framework silicatesAlso framework silicates 25-50% Si is replaced by Al; charge balance maintained by cations K, Na, Ca FELDSPARS Two main groups –Alkali feldspars (KAlSi 3 O 8 – NaAlSi 3 O 8 ), Monoclinic or triclinic –Plagioclase feldspars (NaAlSi 3 O 8 – CaAl 2 Si 2 O 8 ), Triclinic

Alkali feldspars (KAlSi 3 O 8 – NaAlSi 3 O 8 ), Monoclinic or triclinicAlkali feldspars (KAlSi 3 O 8 – NaAlSi 3 O 8 ), Monoclinic or triclinic Complete solid solution between K feldspars & Na feldspars only exists at high temperatures (>700 o )Complete solid solution between K feldspars & Na feldspars only exists at high temperatures (>700 o ) Cooling promotes unmixing & perthite texturesCooling promotes unmixing & perthite textures Perthite where K feldspar phases hosted in Na feldspar phases (or vice versa)Perthite where K feldspar phases hosted in Na feldspar phases (or vice versa) ALKALI FELDSPARS Orthoclase – K end member at high temperatures (monoclinic) Microcline – K end member at low temperatures (triclinic)

ALKALI FELDSPARS IN HAND SPECIMEN Colourless, but can be various colours (e.g. pink) Vitreous lustre Good intersecting cleavages Hardness 6 (I.e. less than quartz) Perthitic fabrics often visible Many show simple/interpenetrative twins Perthitic texture Simple twinning

Plagioclase feldspars (NaAlSi 3 O 8 – CaAl 2 Si 2 O 8 ), triclinicPlagioclase feldspars (NaAlSi 3 O 8 – CaAl 2 Si 2 O 8 ), triclinic Complete solid solution between Na & Ca phases at lower temperatures – no perthite textures are formedComplete solid solution between Na & Ca phases at lower temperatures – no perthite textures are formed Plagioclase series subdivided on the relative proportion of Albite (Ab) and Anorthite end membersPlagioclase series subdivided on the relative proportion of Albite (Ab) and Anorthite end members PLAGIOCLASE FELDSPARS Albite Ab Oligoclase Ab Andesine Ab Labradorite Ab Bytownite Ab Anorthite Ab 10-0

PLAGIOCLASE FELDSPARS IN HAND SPECIMEN Colourless, but can be various colours (often white) Vitreous lustre Good intersecting cleavages Hardness 6 Many show multiple (polysynthetic) twinning No perthitic textures Multiple twinning