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thumb Milk quartz rock Quartz is the most abundant mineral on Earth (about 12% vol.). It has a hexagonal crystal structure made of trigonal-crystallized silica (silicon dioxide, SiO2), with a hardness of 7 on the Mohs scale. Density is 2600kg/m³. The typical shape is a six-sided prism that ends in six-sided pyramids, although these are often distorted, or so massive that only part of the shape is apparent from a mined specimen. Additionally a bed is a common form, particularly for varieties such as amethyst, where the crystals grow up from a matrix and thus only one termination pyramid is present. A quartz geode consists of a hollow rock (usually with an approximately spherical shape) with a core lined with a bed of crystals.
Quartz is one of the world's most common crustal minerals and goes by a bewildering array of different names. The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals visible only under high magnification). Chalcedony is a generic term for cryptocrystalline quartz. The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline.
Although many of the varietal names historically arose from the colour of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Colour is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. This does not always hold true, however.
Major varieties of quartz:
| Variety | Description |
|---|---|
| Chalcedony | Any cryptocrystalline quartz, although generally only used for white or lightly coloured material. Otherwise more specific names are used. |
| Agate | Banded Chalcedony, translucent |
| Onyx | Agate where the bands are straight, parallel and consistent in size. |
| Jasper | Opaque chalcedony, impure |
| AventurineAventurine is a form of quartz, characterised by its translucency and the presence of platy mineral occlusions that give a shimmering or glistening effect termed aventurescence''. The most common colour of aventurine is green, but it may also be orange, b | Translucent chalcedony with small inclusions (usually mica) that shimmer. |
| Tiger's eyeTiger's eye (also Tigers eye Tiger eye is a chatoyant gemstone that is usually yellow- to red-brown, with a silky lustre. It is a fibrous silicified crocidolite; a classic example of pseudomorphous replacement. An incompletely silicified blue variant is c | Fibrous quartz, exhibiting chatoyancyIn gemology, chatoyancy is an optical reflectance effect seen in certain gemstones. Coined from the French, meaning "cat's eye," chatoyancy arises either from the fibrous structure of a material, as in tiger eye quartz, or from fibrous inclusions or cavit. |
| Rock crystal | Clear, colourless. |
| AmethystAmethyst is a violet or purple variety of quartz often used as an ornament. The name is generally said to be derived from the Greek a "not," and methuskein "to intoxicate," expressing the old belief that the stone protected its owner from drunkenness. | Purple, transparent |
| CitrineCitrine also called citrine quartz or citrine topaz is an amber-coloured gemstone. It is a form of quartz with ferric iron impurities. Most commercial citrine is in fact artificially heated amethyst. External link See also: List of minerals Gemstones Mine | Yellow to reddish orange, greenish yellow |
| Rose quartzRose quartz is a type of quartz which exhibits a pale pink to rose red hue. The color is usually considered as due to trace amounts of titanium, iron, or manganese, in the massive material. Some rose quartz contains microscopic rutile needles which produc | Pink, translucent, may display diasterismAsterism has several meanings: In astronomy, it refers to a constellation-like group of stars; see asterism (astronomy) In gemmology, it is an optical phenomenon; see asterism (gemmology) In typography, it refers to a symbol; see asterism (typography). |
| Milk quartz or snow quartz | White, translucent to opaque, may display diasterism |
| Smoky quartz | Brown, transparent |
| Morion | Dark-brown, opaque |
| Carnelian | Reddish orange chalcedony, translucent |
Not all varieties of quartz are naturally occurring. Prasiolite , an olive coloured material, is produced by heat treatment. Although citrine occurs naturally, the majority is the result of heat-treated amethyst. Carnelian is widely heat-treated to deepen its colour.
Because natural quartz is so often twinned , much quartz used in industry is synthesized. Large, flawless and untwinned crystals are produced in an autoclave via the hydrothermal process: emeralds are also synthesized in this fashion.
Quartz occurs in hydrothermal veins and pegmatites. Well-formed crystals may reach several metres in length and weigh hundreds of kilograms. Erosion of pegmatites may reveal expansive pockets of crystals, known as "cathedrals."
Quartz is a common constituent of granite, sandstone and limestone.
Some quartz crystal structures are piezoelectric and are used as oscillators in electronic devices such as quartz clocks and radios.
A amorphous (glass) SiO2, called Lechatelierite , is caused by lightning strikes in sand, distinct from typical window glass that is impure.
The name "quartz" comes from the Greek word Krystallos, meaning "ice". Roman naturalist Pliny the Elder believed quartz to be permanently frozen ice. He supported this idea by saying that quartz is found near glaciers in the Alps and that large quartz crystals were fashioned into spheres to cool the hands. He also knew of ability of quartz to split light into a spectrum. And it was Nicolas Steno's study of Quartz that paved the way for modern crystallography, he discovered that no matter how distorted a quartz crystal the long prism faces always made a perfect 60 degree angle.