Quartz (a-quartz)

Hand Specimen identification: Hardness, lack of cleavage, conchoidal fracture, and vitreous luster usually serve to identify quartz. When euhedral, its pseudohexagonal prismatic habit can be distinctive. Quartz is sometimes confused with calcite, beryl, cordierite, or feldspars. Plates 2.1 through 2.8 show color pictures of quartz.

Chemical Composition :

Hardness : 7

Specific Gravity2.65

Cleavage/Fracture : no cleavage or parting brittle/conchoidal fracture

Luster/Transparency : vitreous/transparent

Color : colorless, white, milky; less commonly purple, pink,yellow, brown, or black

Streak : White

Optical Properties : In thin section, quartz is distinguished by low relief, low birefringence (maximum interference colors are gray), lack of color, lack of cleavage, lack of visible twinning, lack of alteration, usually anhedral charac-ter, and undulatory extinction. Uniaxial 1+ 2; v = 1.544, e = 1.553, d = 0.009. Plates 5.5, 5.6, 5.7, and 5.8 show quartz in thin section

Crystallography : Hexagonal (rhombohedral), a = 4.913, c = 5.405, Z = 3; space group R312 or R322; point group 32.

Habit : Crystals are prismatic or massive. They belong to class 32, but may appear to have 6-fold symmetry. Common crystals are six-sided prisms terminated by rhombohedrons, sometimes appearing to be hexago-nal dipyramids (Figures 1, 2 and 3). Prism faces of-ten show horizontal growth striations. Rare forms in-clude trapezohedra. Most, if not all, quartz is twinned. Two kinds of twins, Dauphinè and Brazil, are common, but normally cannot be seen with the naked eye.

Structure and composition : Quartz is always essentially pure SiO2 but may contain trace amounts of other elements. It consists of a three-dimensional framework of SiO4 tetrahe-dra, with all oxygens shared by two tetrahedra. At 1 atm, upon heating to 573°C, (1,063°F) minor changes in bond angles cause low quartz to change into high quartz (b-quartz) with crystal symmetry 622; it reverts to low quartz when cooled.

Occurrence and associations : Quartz is always essentially pure SiO2 but may contain trace amounts of other elements. It consists of a three-dimensional framework of SiO4 tetrahe-dra, with all oxygens shared by two tetrahedra. At 1 atm, upon heating to 573°C, (1,063°F) minor changes in bond angles cause low quartz to change into high quartz (b-quartz) with crystal symmetry 622; it reverts to low quartz when cooled.

Varieties : Coarsely crystalline varieties of quartz include citrine (yellow to orange; Figure 3), amethyst (purple), rose quartz (pink), smoky quartz (yellow-brown to black), and milky quartz (milky white). Fibrous microcrys-talline varieties include many types of chalcedony, such as carnelian (red), sard (brown), chrysoprase (apple green), agate (banded or variegated), and onyx (white and gray bands). Jasper (iron red), chert (light gray), and flint (dull dark color) are granular microcrystalline varieties of quartz.

Related Minerals : More than a half dozen SiO2 polymorphs exist, the principal ones being low quartz (a-quartz); high quartz (b-quartz); coesite; stishovite; low and high cristobalite; and low, middle, and high tridymite. Low quartz is the only common one. Opal is an amorphous variety of SiO2 that contains some H2O.

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Stishovite

Hand Specimen identification: Stishovite only exists as tiny grains in rocks near meteorite impact craters. …

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