Quartz

 

Quartz

  Quartz is hexagonal and commonly occurs as crystals ranging in size form microscopic to crystals weighing several tons. Where it crystallizes unhindered by other crystals, such as in cavities in rock or in a liquid containing few other crystals, it shows well-developed hexagonal prisms and sometimes showing apparent hexagonal pyramids or dipyramid. When it crystallizes in an environment where growth is inhibited by the surroundings, it rarely show crystal faces. It is also found as microcrystalline masses, such as in the rock chert, and as fibrous masses, such as in chalcedony. Vitreous lusture.

 

As visible crystals, Quartz is one of the more common rock forming minerals. It occurs in siliceous igneous rocks such as volcanic rhyolite and plutonic granitic rocks. It is common in metamorphic rocks at all grades of metamorphism, and is the chief constituent of sand.

Because it is highly resistant to chemical weathering, it is found in a wide variety of sedimentary rocks. Several varieties of Quartz can be found, but these are usually only distinguishable in hand specimen.

 

Rock Crystal - clear Quartz in distinct crystals - usually found growing in open cavities in rock.

 

Amethyst - violet colored Quartz, with the color resulting from trace amounts of Fe in the crystal.

 

Rose Quartz - a pink colored variety, that usually does not show crystal faces, the color resulting from trace amounts of Ti+4.

Smokey Quartz - a dark colored variety that may be almost black, usually forming well-formed crystals. The color appears to result from trace amounts of Al+3 in the structure.

Citrine - a yellow colored variety.

Milky Quartz - a white colored variety with the color being due to fluid inclusions.

Milky Quartz is common in hydrothermal veins and pegmatites.

A fibrous variety of Quartz is called Chalcedony. It is usually brown to gray to translucent with a waxy luster. It is found lining or filling cavities in rock where it was apparently precipitated from an aqueous solution. When it shows bands of color, it is commonly called by the following names:

Carnelian - red colored Chalcedony

Chrysoprase - apple-green colored as a result of coloration from NiO.

 

Agate - alternating curving layers of Chalcedony with different colors or different porosities.

 

Onyx - alternating layers of Chalcedony of different colors or porosities arranged in parallel planes.

 

Bloodstone - green Chalcedony containing red spots of jasper Very fined grained aggregates of cryptocrystalline quartz makes up rock like Flint and Chert.

Flint occurs as nodules in limestone, whereas chert is a layered rock deposited on the ocean floor. The red variety of flint is called Jasper, where the color results from inclusions of hematite.

 

Optical Properties

 

Quartz is uniaxial positive with a low relief and low birefringence, thus exhibited only 1o gray to 1o white interference colors. In thin section it is almost always colorless when viewed without the analyzer inserted. One of its most distinguishing properties in thin section is that it usually has a smooth, almost polished-like surface texture. Quartz is easily distinguished from the Feldspars by the biaxial nature of feldspars, and from Nepheline which is uniaxial negative. Apatite, has similar birefringence to quartz, but is uniaxial negative and has a very high relief.

In Chalcedony, the fibers are usually elongated perpendicular to the c-crystallographic axis and thus are length fast. Normal quartz, when it show an elongated habit, is elongated parallel to the c axis, and is thus length slow.

 

Tridymite

Tridymite is the high temperature polymorph of SiO2. Thus, it is only commonly found in igneous rocks that have been cooled rapidly to surface temperatures, preventing the slow transformation to quartz, the stable form of SiO2 at surface temperatures. Because of this, we only expect to find Tridymite in siliceous volcanic rocks like rhyolites, where it commonly occurs as wedge shaped crystals in cavities in the rock. In volcanic rocks, Tridymite is commonly associated with Cristobalite and Sanidine.

 

Optical Properties

Tridymite usually occurs as orthorhombic or monoclinic wedge shaped crystals with a positive 2V between 40 and 90o. The wedge shape of the crystals is the result of twinning on {110}, and usually as 2 to 3 twinned individuals. Although it has similar birefringence to quartz and feldspar, it has lower refractive indices, and thus shows negative relief compared to quartz and feldspars.

 

Cristobalite

Cristobalite is also a high temperature SiO2 polymorph, and thus has a similar occurrence to Tridymite. It also occurs in thermally metamorphosed sandstones. In volcanic rocks it can occur both as a lining in open cavities, and as fine grained crystals in the groundmass of the rock.

 

Optical Properties

Cristobalite is tetragonal and thus uniaxial. It has a negative optic sign and shows lower relief than quartz, but has similar birefringence.

 

Opal

Opal is amorphous, and thus a mineraloid, with a formula - SiO2 .nH2O..