Origin of Name
From German, quartz, of unknown origin.
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.
Physical Properties
| Hardness | 7 |
| Specific Gravity | 2.65 |
| 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 |
| Fracture | Conchoidal |
| Cleavage | Absent |
Optical Properties
| Colour And Pleochroism | Non-Pleochroic |
| Refarctive Index | Low |
| Birefrigence | Low(0.009) |
| Interference colour | First Order Grey |
| Twinning | Dauphine type, Brazil type, Japanese type |
| Extinction | Wavy |
| Charge | Uniaxial Posetive |
| Relief | Low |
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 character, and undulatory extinction. Uniaxial (+); Womega = 1.544, Epsilon = 1.553, Delta = 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; 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 thombohedrons, sometimes appearing to be hexagonal dipyramids. Prism faces often show horizontal growth striations. Rare forms include trapezohedra. Most, if not all, quartz is twinned, Two kinds of twins, Dauphiné 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 tetrahedra, 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 (beta-quartz) with crystal symmetry 622; it reverts to low quartz when cooled.
3D Model
Occurrence and Associations
Quartz is a common and essential ingredient in many sedimentary, metamorphic, and igneous rocks. It dominates in sandstone and quartzite and occurs in all silicic metamorphic and igneous rocks. It also dominates in beach sands, many soils, and other sediments.
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Varieties
Coarsely crystalline varieties of quartz include citrine (yellow to orange) amethst (purple), rose quartz (pink), smoky quartz (Yellow-brown to black), and milky quartz (milky white). Fibrous microcrystalline 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 (Alpha-quartz); high quartz (Beta-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 SiOz that contains some H2O.