EARTH MATERIALS IIIEARTH MATERIALS III

Rock-forming minerals: silicatesRock-forming minerals: 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

Silicates are dominant mineralsSilicates are dominant minerals

Silicate structureSilicate structure

• Based on Silica tetrahedraSilica tetrahedra• Silica tetrahedra formed by four oxygen ionsfour oxygen ions

surrounding & sharing electrons with silicon ionsilicon ion

• Silicate structure based on repetition of tetrahedra

STRUCTURE of SILICATE MINERALS

Important examplesImportant examples

Olivine, Garnet

PyroxenesAmphibolesMicasQuartz

• Olivines are orthosilicatesorthosilicates• Forsterite Mg2SiO4 – Fayalite Fe2SiO4

• Forms an Isomorphous SeriesIsomorphous Series• Isomorphs – different chemical compositions

but same crystal structure

OLIVINES (Mg,Fe)2 SiO4

No oxygen sharing - No oxygen sharing - tetrahedra bonded to ion tetrahedra bonded to ion (Mg, Fe) between them(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 Also orthosilicatesorthosilicates• Cubic symmetry• Several named varieties, e.g.

• Pyrope (Mg3Al2Si3O12)• Almandine (Fe3Al2Si3O12)

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Almandine

GARNETS IN HAND SPECIMEN

• dark equidimensional crystals• no cleavage• hardness = 6-7.5

PYROXENES

• Single chain silicatesSingle chain silicates• Can be orthorhombic (orthopyroxenes)orthopyroxenes) or

monoclinic (clinopyroxenesclinopyroxenes))• Many named varieties, e.g.:

• Augite – Ca clinopyroxene• Aegirine – Na clinopyroxene• Enstatite – orthopyroxene

Silica tetrahedra linked in chain by shared oxygens, linked Silica tetrahedra linked in chain by shared oxygens, linked laterally by various cations, notably Mg, Fe, Al, Ca, Nalaterally 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 5 - 6

Cleavages in section

Aegirine crystals

AMPHIBOLES• Double chain silicatesDouble chain silicates• Orthorhombic (orthoamphiboles) or monoclinic

(clinoamphiboles)• Many named varieties, e.g.

– Hornblende Ca clinoamphiboleHornblende Ca clinoamphibole– Glaucophane Na clinoamphiboleGlaucophane Na clinoamphibole– Anthophyllite orthoamphiboleAnthophyllite orthoamphibole

Parallel chains joined by shared oxygens, linked laterally by Parallel chains joined by shared oxygens, linked laterally by various cations, notably Mg, Fe, Al, Ca, Navarious 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 KAlMuscovite KAl22(Si(Si33AlOAlO1010)(OH))(OH)22

– Biotite K(Mg,Fe)Biotite K(Mg,Fe)22(Si(Si33AlOAlO1010)(OH))(OH)22

Each tetrahedron linked Each tetrahedron linked to three others by to three others by shared oxygens, sheets shared oxygens, sheets bonded by cations, bonded by cations, notably K, Mg, Fe, Al 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

Muscovite Biotite

CHLORITE

• Sheet silicatesSheet 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 silicatesFramework silicates• QUARTZ (SiO2) most common of the silica

minerals• CHALCEDONY microcrystalline quartz with sub-

microscopic pore spaces filled with water

Each tetrahedron Each tetrahedron shares all its oxygens shares all its oxygens with other silica in with other silica in Quartz (SiOQuartz (SiO22))

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 (KAlSiAlkali feldspars (KAlSi33OO8 8 – –

NaAlSiNaAlSi33OO88), Monoclinic or triclinic), Monoclinic or triclinic

–Plagioclase feldspars (NaAlSiPlagioclase feldspars (NaAlSi33OO8 8

– CaAl– CaAl22SiSi22OO88), Triclinic), Triclinic

• Alkali feldspars (KAlSiAlkali feldspars (KAlSi33OO8 8 – NaAlSi– NaAlSi33OO88), Monoclinic or ), Monoclinic or triclinictriclinic

• Complete solid solution between K feldspars & Na Complete solid solution between K feldspars & Na feldspars only exists at high temperatures (>700feldspars only exists at high temperatures (>700oo))

• Cooling promotes unmixing & Cooling promotes unmixing & perthite perthite texturestextures• PerthitePerthite where K feldspar phases hosted in Na feldspar where K feldspar phases hosted in Na feldspar

phases (or vice versa)phases (or vice versa)

ALKALI FELDSPARS

OrthoclaseOrthoclase – K end – K end member at high member at high temperatures temperatures (monoclinic)(monoclinic)

MicroclineMicrocline – K end – K end member at low member at low temperatures (triclinic)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 (NaAlSiPlagioclase feldspars (NaAlSi33OO8 8 – CaAl– CaAl22SiSi22OO88), triclinic), triclinic• Complete solid solution between Na & Ca phases at Complete solid solution between Na & Ca phases at

lower temperatures – no perthite textures are formedlower temperatures – no perthite textures are formed• Plagioclase series subdivided on the relative Plagioclase series subdivided on the relative

proportion ofproportion of Albite (Ab) Albite (Ab) andand Anorthite Anorthite end membersend members

PLAGIOCLASE FELDSPARS

Albite Ab Albite Ab 100-90100-90

Oligoclase Ab Oligoclase Ab 90-7090-70

Andesine Ab Andesine Ab 70-5070-50

Labradorite Ab Labradorite Ab 50-3050-30

Bytownite Ab Bytownite Ab 30-1030-10

Anorthite AbAnorthite Ab10-010-0

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