Rock forming minerals silicates

EARTH MATERIALS III Rock-forming minerals: silicates Professor Peter Doyle P.doyle@imperial.ac.uk Profdoyle@btinternet.com CLASSIFICATION OF MINERALS Mineral class Example SILICATES CARBONATES SULPHATES OXIDES SULPHIDES HYDROXIDES HALIDES NATIVE Olivine (Mg,Fe)2 SiO4 Calcite CaCO3 Anhydrite CaSO4 Hematite Fe2O3 Pyrite FeS2 Brucite Mg(OH)2 Halite NaCl Gold Au Silicates are dominant minerals Silicate structure • Based on Silica tetrahedra • Silica 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 OLIVINES (Mg,Fe)2 SiO4 • Olivines are orthosilicates • Forsterite Mg2SiO4 – Fayalite Fe2SiO4 • Forms an Isomorphous Series • Isomorphs – different chemical compositions but same crystal structure 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,Fe2+,Mn)3(Al,Fe3+,Cr)2Si3O12 • • • Also orthosilicates Cubic symmetry Several named varieties, e.g • Pyrope (Mg3Al2Si3O12) • Almandine (Fe3Al2Si3O12) Almandine QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture GARNETS IN HAND SPECIMEN • dark equidimensional crystals • no cleavage • hardness = 6-7.5 PYROXENES • Single chain silicates • 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 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 silicates Monoclinic Most common examples: – Muscovite KAl2(Si3AlO10)(OH)2 – Biotite K(Mg,Fe)2(Si3AlO10)(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 usually colourless – Biotite dark brown, colour increases with increasing Fe • Platy, plates flexible with one perfect cleavage • Hardness 2-3 Muscovite Biotite CHLORITE • Sheet silicates with different crystal structure to micas • (Mg,AL,Fe)6(Si,Al)4O10(OH)8 • • • • Monoclinic Typically green, colour increases with increasing Fe Platy, with one perfect cleavage Hardness 2-3 CHLORITE QUARTZ & CHALCEDONY • Framework silicates • QUARTZ (SiO2) most common of the silica minerals • CHALCEDONY microcrystalline quartz with submicroscopic pore spaces filled with water Each tetrahedron shares all its oxygens with other silica in Quartz (SiO2) QUARTZ IN HAND SPECIMEN • Colourless, but many coloured varieties due to chemical impurities (e.g amethyst) • Vitreous lustre • No cleavage – conchoidal fracture • Hardness CHALCEDONY IN HAND SPECIMEN • • • • • Agate - banded variety Flint - dark nodular variety Chert - rock composed of chalcedony Massive with conchoidal fracture Hardness FELDSPARS • Also framework silicates • 25-50% Si is replaced by Al; charge balance maintained by cations K, Na, Ca Two main groups Alkali feldspars (KAlSi3O8 – –Alkali NaAlSi3O8), Monoclinic or triclinic Plagioclase feldspars (NaAlSi3O8 –Plagioclase – CaAl2Si2O8), Triclinic ALKALI FELDSPARS • Alkali feldspars (KAlSi3O8 – NaAlSi3O8), Monoclinic or triclinic • Complete solid solution between K feldspars & Na feldspars only exists at high temperatures (>700o) • Cooling promotes unmixing & perthite textures • Perthite where K feldspar phases hosted in Na feldspar phases (or vice versa) 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 (I.e less than quartz) Perthitic fabrics often visible Many show simple/interpenetrative twins Perthitic texture Simple twinning PLAGIOCLASE FELDSPARS • Plagioclase feldspars (NaAlSi3O8 – CaAl2Si2O8), triclinic • Complete 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 members Albite Ab 100-90 Oligoclase Ab 90-70 Andesine Ab 70-50 Labradorite Ab 50-30 Bytownite Ab 30-10 Anorthite Ab10-0 PLAGIOCLASE FELDSPARS IN HAND SPECIMEN • • • • • • Colourless, but can be various colours (often white) Vitreous lustre Good intersecting cleavages Hardness Many show multiple (polysynthetic) twinning No perthitic textures Multiple twinning [...]... cleavage • Hardness 2-3 Muscovite Biotite CHLORITE • Sheet silicates with different crystal structure to micas • (Mg,AL,Fe)6(Si,Al)4O10(OH)8 • • • • Monoclinic Typically green, colour increases with increasing Fe Platy, with one perfect cleavage Hardness 2-3 CHLORITE QUARTZ & CHALCEDONY • Framework silicates • QUARTZ (SiO2) most common of the silica minerals • CHALCEDONY microcrystalline quartz with submicroscopic... lustre • No cleavage – conchoidal fracture • Hardness 7 CHALCEDONY IN HAND SPECIMEN • • • • • Agate - banded variety Flint - dark nodular variety Chert - rock composed of chalcedony Massive with conchoidal fracture Hardness 7 FELDSPARS • Also framework silicates • 25-50% Si is replaced by Al; charge balance maintained by cations K, Na, Ca Two main groups Alkali feldspars (KAlSi3O8 – –Alkali NaAlSi3O8),... HAND SPECIMEN • • • • Dark coloured but varies depending on Fe/Mg Elongate prismatic habits Intersecting cleavages at 87/93 Hardness 5 - 6 Cleavages in section Aegirine crystals AMPHIBOLES • Double chain silicates • Orthorhombic (orthoamphiboles) or monoclinic (clinoamphiboles) • Many named varieties, e.g – – – Hornblende Ca clinoamphibole Glaucophane Na clinoamphibole Anthophyllite orthoamphibole Parallel... 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 silicates Monoclinic Most common examples: – Muscovite KAl2(Si3AlO10)(OH)2 – Biotite K(Mg,Fe)2(Si3AlO10)(OH)2 Each tetrahedron linked to three others by shared oxygens, sheets bonded by cations, notably ...CLASSIFICATION OF MINERALS Mineral class Example SILICATES CARBONATES SULPHATES OXIDES SULPHIDES HYDROXIDES HALIDES... Anhydrite CaSO4 Hematite Fe2O3 Pyrite FeS2 Brucite Mg(OH)2 Halite NaCl Gold Au Silicates are dominant minerals Silicate structure • Based on Silica tetrahedra • Silica tetrahedra formed by four oxygen... electrons with silicon ion • Silicate structure based on repetition of tetrahedra STRUCTURE of SILICATE MINERALS Important examples Olivine, Garnet Pyroxenes Amphiboles Micas Quartz OLIVINES (Mg,Fe)2