Silica

Silica

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  SILICA MINERALS Quarz,Trydymite and Crystobalite.
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  An introduction tothe rock forming minerals . By. Deer,Howie and Zussman Mineralogy by Berry and Mason Minerals – their constitution and origin By Wenk and Bulakh. Mineralogy by Dexter Perkins Manual of Mineralogy by Klein and Hurlbut
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  Mineral Groups –Silicates Quartz light silicates (pure SiO 2 ) no cleavage (conchoidal fracture) hard, resistant to weathering Quartz
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  With an overallcomposition of Si O2 Quartz is one of the most abundant minerals and occurs as an essential constituent of many igneous, metamorphic and sedimentary rocks. Also found as an accessory mineral and as a secondary mineral in veins and metasomatic deposits. Displays some of the most complex crystal structure.
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  Quartz (mostcommon member)occurs in a trigonal low –temp form ( ∞-quartz- ) and hexagonal high temp form ( b-Quartz); other polymorphs are tridymite, crystabolite, coesite and stishovite. Except of stishovite , silicon is surrounded by four oxygen atoms, forming Si O4 Tetrahedra. The bonds between silicon and oxygen are complex ionic-covalent.
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  1 - CSystems 1. The system SiO 2 Fig. 6-6. After Swamy and Saxena (1994), J. Geophys. Res., 99, 11,787-11,794 . AGU
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  ∞ -quartz- stableat atmospheric temperatures and upto 573°C. b-Quartz- stable from 573-870°C. can exist metastably above 870°C. Tridymite is stable between 870-1470°C. Cristobalite from 1470-1713°C. 1713°C to the boiling point liquid silica is the stable phase.
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  Coesite- a rarehigh pressure phase, produced at 450-800 °C and 38000 atmos. Pressure. Found in rocks subjected to the impact of large meteorites and in ultra high pressure metamorphic rocks. Stishovite- Second high pressure polymorph of Si O2, a high density form of silica, D-4.3, synthesized at 130000atmospheres and >1200°C ; recognized in the Meteor Crater. Arizona. Low Pressure silica minerals having D-2.6 g/cm3. and coesite 2.9gm/cm3
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  Cryptocristalline silica(Chalcedony): Compactvarities containing minute crystals of quartz with submicroscopic pores. Silica Glass(Lechatelierite, vitreous silica): can exist at room temperatures and upto 1000°C when its rate of crystallization rapidly increases. It is an unstable glass at all temperatures below 1713°C
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  Opal- is asolid silica gel containing a large amount of water. It may be colourless, milky white, yellow, red, green blue or black. Opal has composition of SiO2. nH2O., with the water content around 6-10%in precious opal. The structure of precious opal consist of a close-packed array of regular silica spheres: these forms a three dimentional diffraction grating, which is responsible for the play of colours.
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  Varieties of QuarzTwo distinct types (macrocrystlline and cryptocrystalline varities) Macrocrystalline varities. Rock crystal- transparent colourless material Milky: Milk- white and almost opaque, luster often somewhat greasy; occurs in large masses in veins and pegmatites.
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  3. Amethyst: transparentpurple material, usually occurs lining cavities in volcanic rocks (Fe3+). 4. Rose- rose red or pink, rarely in crystals. 5. Yellow (Citrine)- yellow and transparent, Amethyst on heating often turns yellow. 6. Smoky: transparent and semi-transparent brown, gray to nearly black varities. Radiation from radioactive material often develops a smoky appearance in colourless quartz, and this may be responsible for much smoky quartz in nature. Brown and black varieties are due to the traces of alum
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  Brown and blackvarieties are due to the traces of aluminium. Fe 3+ bearing varities are either purple-violet or yellow (citrine). Pink color of some varieties of rose quartz has been due to submicroscopic inclusions of a fibrous nanocrystalline material related to the rare mineral DUMORTIERITE.
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  Cryptocrystlline varieties. Carnelianand Sard: red and reddish brown chalcedony; heliotrope and bloodstone are green chalcedony with redspots. Agate: banded forms of chalcedony. Onyx is plane-banded alternately light and dark agate. Moss Agate: white or cream colored chalcedony enclosing brown or black dendritic moss-like aggregates of manganese oxides. Jasper: opaque chalcedony, generally red, yellow or brown, the colour being due to colloidal particles of iron oxides.
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  Chert and Flint-Massive opaque chalcedony, usually white , pale yellow, gray or black and occuring as nodules or extensive beds in sedimentary rocks. Silicified wood: generally consists of reddish or brown chalcedony.