Quartz
The most abundant mineral in the gemstones family is Quartz . It is found in myriad colors, shapes and varieties . The name quartz derives from the ancient Greek, Krustallos, meaning 'ice' , because the Greek believed that quartz was ice that never melted because it was formed by the gods. Quartz is formed in three types - 1. Igneous rocks 2. Hydrothermal veins 3.Metamorphic
Igneous rocks : Quartz in igneous rocks crystallizes as shapeless grains, but in rhyolites, which have fine grained structure from rapid cooling. In igneous rock well shaped quartz outlines can be seen. Very few grains of quartz are found in igneous rocks. In Brazil there are some granite pegmatites rocks wherein quartz crystals weighing over 600 Kgs have been recorded . In igneous rock massive quartz are found .
Hydrothermal veins : These are the most frequent locations for quartz deposition. These are in general microcrystalline varieties of quartz. Chalcedony of light color without notable banding in commonly found. The deposition of Jaspers and onyx which is also hydrothermal veins quartz. In the state of New York, fine quartz crystals of high luster are found in this category of quartz and these quartz are also known as Herkimer diamonds.
Metamorphic rocks : When heat and pressure act on igneous and sedimentary rocks, metamorphosed equivalents of quartz are formed. Aventurine quartz is found in this variety.
There are two main varieties of quartz :
1. Crystalline quartz : It is the variety in which a range of colors, from clear Rock Crystal to Pink Rose Quartz or purple Amethyst, to dark Smoky quartz. Sometimes minerals rapped within the crystals create a beautiful effects as with Chatoyant quartz or Rutilated quartz. The crystals can be huge up to 22 feet in circumference and weighing 50 to 70 tones.
There are many varieties of crystalline quartz-
Rock crystal, Brown or Smoky quartz, (Brown-Cairngorms) (Black - Morion), Amethyst, Citrine, Ametrine, Rose quartz, Crocidolite, Hawk's-eye, Tiger's- eye, Aventurine quartz, Eosite etc.
2. Cryptocrystalline quartz : In crypto cryptocrystalline quarts the crystals are so small as to be microscopic . In this group includes chalcedony, Agate, Jasper, Onyx, Carnelian. Chrysoprase , bloodstone, sard and petrified wood. Prase, Plasma ,Myrickite, Lace Agate , fire Agate, Banded Agate.
Deposits
Deposits are found in Brazil, Alps, United States, Madagascar, Mexico, Spain, South Africa, India, Japan, Russia, Argentina, Burma(Myanmar), Scotland, Austria(Salzburg), Tanzania, Saxony(Germany), Finland, Namibia, Uruguay, Zimbabwe, Srilanka.
Properties
Quartz(Crystalline)
| Species | Quartz. |
|---|---|
| Variety | Amethyst-Purple, Citrine-Yellow, Cairngorm-brown, Morion-Smoky, Aventurine-Green,Colorless-Rock Crystal, Pink- Rose Quartz, Quartz Cat's Eye; |
| Chem. Comp. | Sio2(Silicon Dioxide) |
| Crystal System | Trigonal |
| Habit | Prismatic. |
| Colour | Violet to Purple, Grey brown or Yellowish Brown, Yellow, Orange, Orange-Brown to red orange. |
| Cause of color | Amethyst colour centre due to iron. Citrine- Iron. Smoky- Aluminium impurity. |
| Rose -Titanium and manganese. Rock Crystal- Scattering of light. | |
| Aventurine- Green fuchsite inclusions. Blue-Blue dumortierite inclusions and crocidolite | |
| Transparency | Transparent to opaque. |
| Lustre | Vitreous |
| Hardness | 7 |
| S.G. | 2.65 |
| Cleavage | Imperfect |
| Fracture | Conchoidal |
| SR/DR | DR |
| Pleochroism | Weak to Strong |
| Optic Sign | U + Ve |
| Optic Char | Uniaxial |
| R.I. Range | 1.544-1.553 |
| Birefringence | 0.009 |
| U.V. Light | Inert |
Quartz-Cat's Eye(Crystalline)
| Species | Quartz. |
|---|---|
| Variety | Quartz cat's eye, Tiger's Eye-Crocidolite(Golden Brown), Rutilated quartz, Hawk's Eye(blue). |
| Chem. Comp. | Sio2(Silicon Dioxide) |
| Crystal System | Trigonal |
| Habit | Aggregate-Fibrous |
| Colour | Gray, Brown, Green or Greenish-Yellow, Blue. |
| Cause of color | Golden, Yellow And Blue - due to iron of different valency. |
| Transparency | Translucent to opaque. |
| Lustre | Vitreous |
| Hardness | 7 |
| S.G. | 2.65 |
| Cleavage | None |
| Fracture | Conchoidal |
| SR/DR | AGG |
| Pleochroism | Weak |
| Optic Sign | U + Ve |
| Optic Char | Uniaxial |
| R.I. Range | 1.553-1.544 |
| Birefringence | 0.009 |
| U.V. Light | Inert. |
Quartz(Cryptocrystalline)
| Species | QUARTZ. |
|---|---|
| Variety | Chalcedony-Unbanded, carnelian-red, Chryoprase-Apple Green, Agate-Concentric |
| Chem. Comp. | Sio2(Silicon Dioxide) |
| Crystal System | Trigonal |
| Habit | Prismatic. |
| Colour | Violet to Purple, Grey brown or Yellowish Brown, Yellow, Orange, Orange-Brown to red |
| Cause of color | Amethyst colour centre due to iron. Citrine- Iron. Smoky-Aluminium impurity. Rose- |
| Transparency | Transparent to opaque. |
| Lustre | Vitreous |
| Hardness | 7 |
| S.G. | 2.65 |
| Cleavage | Imperfect |
| Fracture | Conchoidal |
| SR/DR | DR |
| Pleochroism | Weak to Strong |
| Optic Sign | U + Ve |
| Optic Char | Uniaxial |
| R.I. Range | 1.544-1.553 |
| Birefringence | 0.009 |
| U.V. Light | Inert. |
Inclusion
Colour zoning, liquid and 2 phase inclusions, rutile.
Aventurine quartz - Green fuchsite inclusions.
Blue quartz - Blue dumortierite inclusions.
Crystalline quartz - Fibres of asbestos.
Cryptocrystalline quartz - bands, moss - like, fibres.
Simulants
Paste, synthetic quartz, scapolite, topaz, corundum, cat's eye of chrysoberyl, sillimanite, apatite, scapolite, tourmaline, crystalline quartz, serpentine, feldspar, glass, fluorite, chrysoberyl cat's eye.
