ERTH 2002

1
ERTH 2002

• Mineral of the Week (MoW)
• olivine
• 2. pyroxene
• 3. feldspar (some review)
• 4. garnet
• 5. amphibole
• 6. mica
• 7. quartz
• 8. clays
• 9. carbonates (some review)
• 10. Fe-Ti oxides
• 11. sulphides (some review)

properties chemical composition crystal
structure phase diagrams occurrence use
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MoW 7 Silica Group (Nesse, Ch. 12, p. 201-208)
amethyst

quartz
smoky quartz
opal
photos mindat.org
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MoW 7 Silica Group (Nesse, Ch. 12, p. 201-208)
a framework silicate with the formula SiO2 by
far the most abundant silica group mineral
is a-quartz other SiO2 polymorphs are preserved
in the geological record under special
circumstances a number of non- or
micro-crystalline varieties of SiO2 also form
important minerals
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MoW 7 Silica Group (Nesse, Ch. 12, p. 201-208)
b-quartz (hexagonal)
right-handed left-handed spirals
interference colour range
key optical properties PPL low relief,
clarity XN low birefringence uniaxial
ve
a-quartz (trigonal)
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MoW 7 Silica Group (Nesse, Ch. 12, p. 201-208)
quartz in thin section
204 quartz arenite (2.5 mm)
OM-1 vein quartz (6.25 mm)
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MoW 7 Silica Group (Nesse, Ch. 12, p. 201-208)
phase diagram for SiO2 system
certain SiO2 polymorphs indicate specific P-T
ranges and geological environments
ultra-high pressure (UHP) conditions
ultra-high temperature (UHT) conditions
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MoW 7 Silica Group (Nesse, Ch. 12, p. 201-208)
phase diagram for SiO2 system
restricted to impact craters
Coesite diagnostic of UHP metamorphism (continenta
l subduction)
a-quartz ?? b-quartz transformation always takes
place b-quartz not stable at surface
restricted to volcanic environments
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MoW 7 Silica Group (Nesse, Ch. 12, p. 201-208)
non- or micro-crystalline varieties of SiO2
microcrystalline moganite alternating
sheets of right- and left-handed a-quartz
chert (including jasper, flint) microcrystalline,
granular chalcedony (including
agate) microcrystalline, fibrous
non-crystalline opal - a form of colloidal
silica - important gemstone
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MoW 7 Silica Group (Nesse, Ch. 12, p. 201-208)
occurrence very common in continental
crust - felsic to intermediate igneous rocks -
clastic sedimentary rocks - hydrothermal vein
deposits and their metamorphic
equivalents use important industrial and
construction material - glass, ceramics -
elemental Si (electronics, caulking,
lubricant) - aggregate, abrasives, decorative -
semiprecious gemstone (opal, agate, amethyst, etc)