Al2SiO5 Group of Minerals

Sillimanite - OrthorhombicMullite - OrthorhombicAndalusite - OrthorhombicKyanite - Triclinic

Sillimanite – Al2O3.SiO2• α 1.654-1.661• β 1.658-1.662• 1.673-1.683• 0.020-0.022• Optic axial angle (2V) 21o - 30o

• RI : =x, =y, =z (least, intermediate and greatest)• O.A.P (010)• Dispersion: red > violet, strong, D: 3.23-3.27, H: 7.5-6.5• Cleavages: {010} good, • Colour: Colourless to white, yellow, brown, greyish green• Colourless in thin section• Pleochroism: = pale brown or pale yellow, =dark brown or

blue, β=brown or greenish

Sillimanite - Structure7.59Å

7.44Å

C=5.75Å, Z=4

Chemistry

• Al2O3. SiO2 = Al2SiO5

• Fe+3 is the common ion replacing Al+3

• Synthesized from its component oxides• 300±50oC, 8±0.5kb• At 1545oC, unstable, converted to mullite + liquid• Alteration products-muscovite, sericite,

pyrophyllite, kaolinite, montmorillonite• Can be converted into Kyanite

Optical and Physical Properties

• Long prismatic crystals or as fibrous mat of fine crystalline material

• Relief is moderately high• RI vary slightly, birefringence is comparatively

strong• Positive (length-slow) elongation

Paragenesis

• Higher grades of thermally metamorphosed argillaceous rocks

• Sillimanite-cordierite gneiss and biotite-sillimanite hornfels

• Regional metamorphism of pelitic rocks followed by thermal metamorphism

Mullite - 3Al2O3.2SiO2• α 1.640-1.670• β 1.642-1.675• 1.651-1.690• 0.012-0.028• Optic axial angle (2V) 45o - 61o

• RI : =x, =y, =z (least, intermediate and greatest)• O.A.P (010)• Dispersion: red > violet, strong, D: 3.15-3.26, H: 7-6• Cleavages: {010} distinct, • Colour: Colourless to white, yellow, pink or red, colourless or pinkish in

thin section• Pleochroism: = β colourless, =pinkish

Mullite Structure

a= 7.53-7.59b=7.67-7.73c=2.884-2.903Å

Chemistry

• Deviation of composition with 60 mol% Al2O3

• Synthetic mullite 2Al2O3.SiO2 or 67 mol% Al2O3 has been produced

• Fe+3 and Ti may replace Al

Optical and physical properties

• RI can be raised by increasing Al2O3 and substitution of Fe+3

• Iron mullite with Fe2O : 3 5.93, TiO2 : 0.55 =1.690

Paragenesis

• Pelitic xenoliths (buchites) in basic igneous rocks

• Iron mullite thermally metamorphosed lateritic lithomarge

• Common refractory produce

Andalusite – Al2O3.SiO2• α 1.629-1.649• β 1.633-1.653• 1.638-1.660• 0.009-0.011• Optic axial angle (2V) 73o - 86o

• RI : =z, =y, =x (least, intermediate and greatest)• O.A.P (010)• Dispersion: red < violet, D: 3.13-3.16, H: 7.5-6.5• Cleavages: {110} good, • Colour: pink or red rose, grey violet yellow, green, or white, • Colourless in thin section, but pink or green• Pleochroism: in coloured varities weak, = rose pink, β & =greenish yellow

Andalusite - Structure

a=7.78, b=7.92, c=5.57Å Z=4

Chemistry

• Andalusite is relatively pure with Al2SiO5

• Ferric and manganese – replaceable ions• Mn-rich variety – Manganandalusite (7% Mn2O3)• Viridine – green variety of andalusite contain 9.6%

Fe2O3, 7.6% Mn

• Synthesized from kaolinite or from Al2O3+SiO2 at 450-650oC, 5-8 kb

• Alter to sericite, sillimanite, kyanite

Optical and physical properties

• RI & Sp.gr. Increased by Ferric and manganese ions

• Pleochroism related to Fe and Mn content• Pink & red varieties contain Fe• Green varities contain Mn• 1450-1500oC andalusite converted to mullite• Used in refractories

Distinguishing feature

• Square cross section• High relief• Low birefringence• Length fast

Paragenesis• Typically in argillaceous rocks of contact

aureoles around igneous intrusions• Often associated with cordierite• Common detrital mineral in some sandstones

Gibbs' phase rule

Kyanite – Al2O3.SiO2• α 1.712-1.718• β 1.721-1.723• 1.727-1.734• 0.012-0.016• Optic axial angle (2V) 82o - 83o

• Dispersion: red > violet, weak, D: 3.53-3.65, H: 5.5-7• Cleavages: {100} perfect, {010} good • Colour: Blue to white, grey, green, yellow, pink or black• Colourless to pale blue in thin section• Pleochroism: weak, = colourless, =cobalt blue, β=violet blue

Kyanite - Structurea=7.10,b=7.74,c=5.57Å,=90o5.5’,=101o2’,=105o44.5’,vol. 15.3Å Slighlty distortedcubic array

Chemistry

• Limited amount Fe+3 can enter structure• Cr sometimes present in moderate amounts• Synthesized at 900oC, 20 kb• Alteration products – pyrophyllite, muscovite,

sericite

Optical and physical properties

• Relief is distinctly high• Colourless in thin section• Birefringence is moderate, higher orders of 1st

order colours• Extinction angle 30o to 0o

• OAP is perpendicular to {100}

Paragenesis

• Typical mineral of regional metamorphism of pelitic or rarely psammitic rocks

• Kyanite derived from pyrophyllite by dehydration of paragonite with addition of quartz

• In thermal aureoles with staurolite• Found in eclogites and amphibolites• Also found in pegmatites by quartz-kyanite

segregation veins • Common detrital mineral in sedimentary rocks