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Aluminosilicate Minerals

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Aluminosilicate Minerals 3 polymorphs of Al2SiO5 are important metamorphic minerals Topaz Aluminosilicate mineral as well, one oxygen substituted with OH, F Al2SiO4 ... – PowerPoint PPT presentation

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Title: Aluminosilicate Minerals


1
Aluminosilicate Minerals
  • 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).

Andalusite
Kyanite
Sillimanite
  • 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.

2
Aluminosilicate Minerals
  • 3 polymorphs of Al2SiO5 are important metamorphic
    minerals

Andalusite
Kyanite
Sillimanite
3
Topaz
  • Aluminosilicate mineral as well, one oxygen
    substituted with OH, F
  • Al2SiO4(F,OH)2
  • Where do you think Topaz forms??

4
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 serpentine brucite
  • Asbestosform variety is chrysotile (accounts for
    95 of worlds asbestos production ? MUCH LESS
    DANGEROUS than crocidolite)

5
Phyllosilicates
T O - T O - T O
Yellow (OH)
vdw
Serpentine Mg3 Si2O5 (OH)4 T-layers and
triocathedral (Mg2) layers (OH) at center of
T-rings and fill base of VI layer ?
vdw
weak van der Waals bonds between T-O groups
6
Serpentine
Antigorite maintains a sheet-like form by
alternating segments of opposite curvature
Chrysotile does not do this and tends to roll
into tubes
7
Serpentine
Veblen and Busek, 1979, Science 206, 1398-1400.
S serpentine T talc
Nagby and Faust (1956) Am. Mineralogist 41,
817-836.
The rolled tubes in chrysotile resolves the
apparent paradox of asbestosform sheet silicates
8
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

9
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

10
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

11
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

12
Epidote Group
  • Sorosilicates (paired silicate tetrahedra)
  • Include the mineral Epidote Ca2FeAl2Si3O12(OH),
    Zoisite (Ca2Al3Si3O12(OH) and clinozoisite
    (polymorph)

13
  • Garnets

Garnet A23 B32 SiO43 Pyralspites - B
Al Pyrope Mg3 Al2 SiO43 Almandine Fe3 Al2
SiO43 Spessartine Mn3 Al2 SiO43
Ugrandites - A Ca Uvarovite Ca3 Cr2
SiO43 Grossularite Ca3 Al2 SiO43
Andradite Ca3 Fe2 SiO43 Occurrence Mostly
metamorphic Some high-Al igneous Also in some
mantle peridotites
Garnet (001) view blue Si purple A
turquoise B
14
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

15
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
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