Nesosilicates
"Nesosilicates" or "Orthosilicates": island silicates - SiO4
single tetrahedra bonded together by cations that sit in octahedral sites:
Si:O ratio is 1 : 4
Examples of Nesosilicates:
Easily weathered:
Olivine (Fe,Mg)2SiO4
Garnet (Ca,Fe,Mg)3(Al,Cr,Fe)2(SiO4)3
Resistant to weathering
Topaz Al2SiO4(OH,F)2
Kyanite, Andalusite, Sillimanite (polymorphs):
Al2SiO5 = Al2·SiO4
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kyanite
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andalusite
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sillimanite
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high press
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low temp
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high temp
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- form under different temp. and press. conditions, thus
good indicators of formational environment
In olivine and garnet: cleavage is irregular because the isolated tetrahedral structure
produces no preferred plane of weakness
Olivine alters to serpentine (a phyllosilicate)
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