Rock forming minerals silicates

EARTH MATERIALS IIIRock-forming minerals: silicates Professor Peter Doyle P.doyle@imperial.ac.uk Profdoyle@btinternet.com... STRUCTURE of SILICATE MINERALSImportant examples Olivine, Gar

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 Olivine (Mg,Fe)2 SiO4 CARBONATES Calcite CaCO3

SULPHATES Anhydrite CaSO4

OXIDES Hematite Fe2O3

SULPHIDES Pyrite FeS2

HYDROXIDES Brucite Mg(OH)2

Silicates are dominant minerals

Silicate structure

• Silica tetrahedra formed by four oxygen ions 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 are orthosilicates

• Forsterite Mg 2 SiO 4 – Fayalite Fe 2 SiO 4

• Forms an Isomorphous Series

• Isomorphs – different chemical compositions

but same crystal structure

OLIVINES (Mg,Fe)2 SiO4

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

Almandine

GARNETS IN HAND SPECIMEN

• dark equidimensional crystals

• no cleavage

• hardness = 6-7.5

• Single chain silicates

• Can be orthorhombic ( orthopyroxenes) or

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

• Orthorhombic (orthoamphiboles) or monoclinic

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

HORNBLENDE

ACTINOLITE

GLAUCOPHANE

• Sheet silicates

• Monoclinic

• Most common examples:

– 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 usually colourless Muscovite

– Biotite dark brown, colour increases with increasing Fe Biotite

• Platy, plates flexible with one perfect cleavage

• Hardness 2-3

• 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

QUARTZ & CHALCEDONY

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

• Agate - banded variety

• Flint - dark nodular variety

• Chert - rock composed of chalcedony

• Massive with conchoidal fracture

• Hardness 7

• Also 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

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 ), triclinic

lower temperatures – no perthite textures are formed

proportion of Albite (Ab) and Anorthite end members

PLAGIOCLASE FELDSPARS IN HAND SPECIMEN

• Colourless, but can be various colours (often white)