SILICA MINERALS Quarz, Trydymite and Crystobalite.
<ul><li>An introduction to the rock forming minerals . By. Deer,Howie and Zussman </li></ul><ul><li>Mineralogy by Berry an...
 
 
Mineral Groups – Silicates Quartz light silicates (pure SiO 2 ) no cleavage (conchoidal fracture) hard, resistant to weath...
<ul><li>With an overall composition of Si O2 </li></ul><ul><li>Quartz is one of the most abundant minerals and occurs as a...
<ul><li>Quartz  (most common member)occurs in a trigonal low –temp form ( ∞-quartz-  ) and hexagonal high temp form ( b-Qu...
1 - C Systems 1. The system SiO 2 Fig. 6-6.  After Swamy and Saxena (1994),  J. Geophys. Res., 99, 11,787-11,794 . AGU
<ul><li>∞ -quartz- stable at atmospheric temperatures and upto 573°C. </li></ul><ul><li>b-Quartz- stable from 573-870°C. c...
<ul><li>Coesite- a rare high pressure phase, produced at 450-800 °C and 38000 atmos. Pressure. Found in rocks subjected to...
<ul><li>Cryptocristalline silica(Chalcedony): Compact varities containing minute crystals of quartz with submicroscopic po...
<ul><li>Opal- is a solid silica gel  containing a  large amount of water. It may be  colourless, milky white, yellow, red,...
Varieties of Quarz <ul><li>Two distinct types (macrocrystlline and cryptocrystalline varities) </li></ul><ul><li>Macrocrys...
<ul><li>3. Amethyst: transparent purple material,  usually occurs lining cavities in volcanic rocks (Fe3+). </li></ul><ul>...
<ul><li>Brown and black varieties are due to the traces of aluminium. </li></ul><ul><li>Fe 3+ bearing varities are either ...
<ul><li>Cryptocrystlline varieties. </li></ul><ul><li>Carnelian and Sard: red and reddish brown chalcedony; heliotrope and...
<ul><li>Chert and Flint- Massive opaque chalcedony, usually white , pale yellow, gray or black and occuring as nodules or ...
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Silica

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Silica

  1. 1. SILICA MINERALS Quarz, Trydymite and Crystobalite.
  2. 2. <ul><li>An introduction to the rock forming minerals . By. Deer,Howie and Zussman </li></ul><ul><li>Mineralogy by Berry and Mason </li></ul><ul><li>Minerals – their constitution and origin </li></ul><ul><li>By Wenk and Bulakh. </li></ul><ul><li>Mineralogy by Dexter Perkins </li></ul><ul><li>Manual of Mineralogy by Klein and Hurlbut </li></ul>
  3. 5. Mineral Groups – Silicates Quartz light silicates (pure SiO 2 ) no cleavage (conchoidal fracture) hard, resistant to weathering Quartz
  4. 6. <ul><li>With an overall composition of Si O2 </li></ul><ul><li>Quartz is one of the most abundant minerals and occurs as an essential constituent of many igneous, metamorphic and sedimentary rocks. </li></ul><ul><li>Also found as an accessory mineral and as a secondary mineral in veins and metasomatic deposits. </li></ul><ul><li>Displays some of the most complex crystal structure. </li></ul>
  5. 7. <ul><li>Quartz (most common member)occurs in a trigonal low –temp form ( ∞-quartz- ) and hexagonal high temp form ( b-Quartz); other polymorphs are tridymite, crystabolite, coesite and stishovite. </li></ul><ul><li>Except of stishovite , silicon is surrounded by four oxygen atoms, forming Si O4 Tetrahedra. </li></ul><ul><li>The bonds between silicon and oxygen are complex ionic-covalent. </li></ul>
  6. 8. 1 - C Systems 1. The system SiO 2 Fig. 6-6. After Swamy and Saxena (1994), J. Geophys. Res., 99, 11,787-11,794 . AGU
  7. 9. <ul><li>∞ -quartz- stable at atmospheric temperatures and upto 573°C. </li></ul><ul><li>b-Quartz- stable from 573-870°C. can exist metastably above 870°C. </li></ul><ul><li>Tridymite is stable between 870-1470°C. </li></ul><ul><li>Cristobalite from 1470-1713°C. </li></ul><ul><li>1713°C to the boiling point liquid silica is the stable phase. </li></ul>
  8. 10. <ul><li>Coesite- a rare high 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. </li></ul><ul><li>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. </li></ul><ul><li>Low Pressure silica minerals having D-2.6 g/cm3. and coesite 2.9gm/cm3 </li></ul>
  9. 11. <ul><li>Cryptocristalline silica(Chalcedony): Compact varities containing minute crystals of quartz with submicroscopic pores. </li></ul><ul><li>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 </li></ul>
  10. 12. <ul><li>Opal- is a solid silica gel containing a large amount of water. It may be colourless, milky white, yellow, red, green blue or black. </li></ul><ul><li>Opal has composition of SiO2. nH2O., with the water content around 6-10%in precious opal. </li></ul><ul><li>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. </li></ul>
  11. 13. Varieties of Quarz <ul><li>Two distinct types (macrocrystlline and cryptocrystalline varities) </li></ul><ul><li>Macrocrystalline varities. </li></ul><ul><li>Rock crystal- transparent colourless material </li></ul><ul><li>Milky: Milk- white and almost opaque, luster often somewhat greasy; occurs in large masses in veins and pegmatites. </li></ul>
  12. 14. <ul><li>3. Amethyst: transparent purple material, usually occurs lining cavities in volcanic rocks (Fe3+). </li></ul><ul><li>4. Rose- rose red or pink, rarely in crystals. </li></ul><ul><li>5. Yellow (Citrine)- yellow and transparent, Amethyst on heating often turns yellow. </li></ul><ul><li>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. </li></ul><ul><li>Brown and black varieties are due to the traces of alum </li></ul>
  13. 15. <ul><li>Brown and black varieties are due to the traces of aluminium. </li></ul><ul><li>Fe 3+ bearing varities are either purple-violet or yellow (citrine). </li></ul><ul><li>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. </li></ul>
  14. 16. <ul><li>Cryptocrystlline varieties. </li></ul><ul><li>Carnelian and Sard: red and reddish brown chalcedony; heliotrope and bloodstone are green chalcedony with redspots. </li></ul><ul><li>Agate: banded forms of chalcedony. Onyx is plane-banded alternately light and dark agate. </li></ul><ul><li>Moss Agate: white or cream colored chalcedony enclosing brown or black dendritic moss-like aggregates of manganese oxides. </li></ul><ul><li>Jasper: opaque chalcedony, generally red, yellow or brown, the colour being due to colloidal particles of iron oxides. </li></ul>
  15. 17. <ul><li>Chert and Flint- Massive opaque chalcedony, usually white , pale yellow, gray or black and occuring as nodules or extensive beds in sedimentary rocks. </li></ul><ul><li>Silicified wood: generally consists of reddish or brown chalcedony. </li></ul>

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