Title: Aluminosilicate Minerals
1Aluminosilicate 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.
2Aluminosilicate Minerals
- 3 polymorphs of Al2SiO5 are important metamorphic
minerals
Andalusite
Kyanite
Sillimanite
3Topaz
- Aluminosilicate mineral as well, one oxygen
substituted with OH, F - Al2SiO4(F,OH)2
- Where do you think Topaz forms??
4Serpentine 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)
5Phyllosilicates
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
6Serpentine
Antigorite maintains a sheet-like form by
alternating segments of opposite curvature
Chrysotile does not do this and tends to roll
into tubes
7Serpentine
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
8Chlorite
- 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
9Prehnite-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
10Micas
- 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
11Zeolites
- 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
12Epidote Group
- Sorosilicates (paired silicate tetrahedra)
- Include the mineral Epidote Ca2FeAl2Si3O12(OH),
Zoisite (Ca2Al3Si3O12(OH) and clinozoisite
(polymorph)
13Garnet 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
14Staurolite
- 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
15Metamorphic 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