Aluminosilicate Minerals

Andalusite Kyanite Sillimanite

• SILLIMANITE: Orthorhombic: Octahedral Al chains (6-fold) are crosslinked by both Si and Al tetrahedra (4-fold).

• ANDALUSITE: Orthorhombic: 5-coordinated Al; Same octahedral (6-fold) chains.

• KYANITE: Triclinic: All the Al is octahedrally coordinated (6- and 6-fold).

•Clearly, changes in structure are in response to changing P and T. Result is changes in Al coordination. •Phase transformations require rebonding of Al. Reconstructive polymorphism requires more energy than do displacive transformations. Metastability of these 3 are therefore important (Kinetic factors limit equilibrium attainment).•All 3 are VERY important metamorphic index minerals.

Aluminosilicate Minerals

• 3 polymorphs of Al2SiO5 are important metamorphic minerals

Andalusite Kyanite Sillimanite

Topaz• Aluminosilicate mineral as well, one oxygen

substituted with OH, F

• Al2SiO4(F,OH)2

• Where do you think Topaz forms??

Serpentine Minerals

• Mg3Si2O5(OH)4 minerals (principally as antigorite, lizardite, chrysotile polymorphs)

• Forms from hydration reaction of magnesium silicates– Mg2SiO4 + 3 H2O Mg3Si2O5(OH)4 + Mg(OH)2

forsterite serpentinebrucite

• Asbestosform variety is chrysotile (accounts for 95% of world’s asbestos production MUCH LESS DANGEROUS than crocidolite)

Phyllosilicates

Serpentine:Serpentine: Mg Mg33 [Si [Si22OO55] (OH)] (OH)44

T-layers and T-layers and tritriocathedral (Mgocathedral (Mg2+2+) layers ) layers

(OH) at center of T-rings and fill base of VI layer (OH) at center of T-rings and fill base of VI layer

Yellow = (OH)Yellow = (OH)

T T O O -- T T O O -- T T OO

vdwvdw

vdwvdw

weak van der Waals bonds between T-O groups weak van der Waals bonds between T-O groups

Serpentine

Octahedra are a bit larger than tetrahedral Octahedra are a bit larger than tetrahedral match, so they cause bending of the T-O match, so they cause bending of the T-O layers (after Klein and Hurlbut, 1999).layers (after Klein and Hurlbut, 1999).

Antigorite maintains a Antigorite maintains a sheet-like form by sheet-like form by

alternating segments of alternating segments of opposite curvatureopposite curvature

Chrysotile does not do this Chrysotile does not do this and tends to roll into tubesand tends to roll into tubes

Serpentine

The rolled tubes in chrysotile resolves the apparent The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicatesparadox of asbestosform sheet silicates

S = serpentine T = talcS = serpentine T = talcNagby and Faust (1956) Am. Mineralogist 41, 817-836.

Veblen and Busek, 1979, Science 206, 1398-1400.

Chlorite• Another phyllosilicate, a group of difficult

to distinguish minerals• Typically green, and the dominant and

characteristic mineral of greenschist facies rocks

• Forms from the alteration of Mg-Fe silicates (pyroxenes, amphiboles, biotite, garnets)

• Clinochlore, chamosite, pennantite, nimmite – end members• Chloritoid - Similar in appearance to

chlorite, but different 2V and relief

Prehnite-Pumpellyite

• Low-grade metamorphic minerals

• Minerals related to chlorite, form at slightly lower P-T conditions

• Prehnite is also green, pumpellyite green too, varies based on Fe content

• Prehnite + chlorite pumpellyite + quartz

Micas• Biotite and Muscovite are also important

metamorphic minerals (muscovite often the principle component of schists)

• Phlogopite – similar to biotite, but has little iron, forms from Mg-rich carbonate deposits and a common mineral in kimberlites (diamond-bearing material)

• Sericite – white mica (similar to muscovite) – common product of plagioclase feldspar alteration at low grades

Zeolites• Diverse group of minerals forming at lower

metamorphic grades• Framework silicas, but characteristically

containing large voids and highly variable amounts of H2O

– Name is from the greek – meaning to boil stone as the water can de driven off with heat

– Voids can acts as molecular sieves and traps for many molecules

– Diversity of minerals in this group makes a for a wide variety of sieve and trapping properties selective for different molecules

Epidote Group

• Sorosilicates (paired silicate tetrahedra)

• Include the mineral Epidote Ca2FeAl2Si3O12(OH), Zoisite (Ca2Al3Si3O12(OH) and clinozoisite (polymorph)

Garnets

Garnet (001) view blue = Si purple = A turquoise = BGarnet (001) view blue = Si purple = A turquoise = B

Garnet: AGarnet: A2+2+33 B B3+3+

22 [SiO [SiO44]]3 3

““Pyralspites”Pyralspites” - B = Al - B = AlPyPyrope: Mgrope: Mg33 Al Al22 [SiO [SiO44]]3 3

AlAlmandine: Femandine: Fe33 Al Al22 [SiO [SiO44]]33

SpSpessartine: Mnessartine: Mn33 Al Al22 [SiO [SiO44]]33

““Ugrandites”Ugrandites” - A = Ca - A = CaUUvarovite: Cavarovite: Ca33 Cr Cr22 [SiO [SiO44]]33

GrGrossularite: Caossularite: Ca33 Al Al22 [SiO [SiO44]]33

AndAndradite: Caradite: Ca33 Fe Fe22 [SiO [SiO44]]33

Occurrence:Occurrence:Mostly metamorphicMostly metamorphicSome high-Al igneousSome high-Al igneousAlso in some mantle peridotitesAlso in some mantle peridotites

Staurolite• Aluminosilicate - Fe2Al9Si4O22(OH)2

• Similar structure to kyanite with tetrahedrally coordinated Fe2+ easily replaced by Zn2+ and Mg2+

• Medium-grade metamorphic mineral, typically forms around 400-500 C– chloritoid + quartz = staurolite + garnet– chloritoid + chlorite + muscovite = staurolite + biotite +

quartz + water

• Degrades to almandine (garnet at higher T)– staurolite + muscovite + quartz = almandine +

aluminosilicate + biotite + water

Metamorphic chain silicates

• Actinolite and tremolite are chain silicates derived from dolomite and quartz and common in low-mid grade metamorphic rocks

• Riebeckite and Glaucophane are also chain silicates – higher grade minerals, often a blue color

• These minerals usually lower P, higher T conditions