Quartz

Quartz is a very common mineral, a chemical compound of silicon and oxygen, silicon dioxide SiO2, commonly called silica. If pure, quartz is a colorless, transparent, and very hard crystalline material of glass-like look. The well-known rock crystals - six-sided prisms with a six-sided pyramid at their ends - are simply well formed crystals of quartz.

Quartz appears in a number of colored varieties, like amethyst (violet), citrine (yellow), or smoky quartz (gray, brown to black). It also occurs in dense forms with no visible crystals, like the multi-colored agate and the gray flint.

Quartz is an important rock-forming mineral, being a constituent of many common rocks, like granite. The word "Quarz" (the "t" is missing on purpose) is known from European literature on mining dating back to the 14th century. It is probably of German or Slavic origin.

Quartz Varieties

Quartz occurs in a great number of varieties that differ in form and color. It occurs as massive aggregates, dense nodules, or crystals in druses. Quartz is colorless if pure, but may assume any color due to inclusions of other minerals or built-in trace elements.

The more common of the varieties have been given their own names - on these web pages about 25 different varieties are distinguished. The different forms of quartz are usually classified in 2 big groups:

• Macro-crystalline varieties are those that form crystals, like amethyst, or have a macroscopical crystalline structure. When people talk about "quartz", they usually think of macro-crystalline quartz.

  . Rock Crystal	. Citrine	. Prase	. Rose Quartz
  Smoky Quartz	Pink Quartz	Ametrine	Ferruginous Quartz
  Amethyst	Prasiolite	Milky Quartz	Tiger's Eye
  Aventurine .	Cat's Eye .	Blue Quartz .	Hawk's Eye .

• Crypto-crystalline or microcrystalline varieties that do not show any visible crystals and have a dense structure, like agate. Cryptocrystalline varieties are sometimes grouped together under the term chalcedony.

  . Chalcedony	. Agate	. Carnelian	. Heliotrope
  Flint	Plasma	Chrysoprase	Chert
  Jasper	Sard	Onyx

If you know what tiger's eye and aventurine look like, you might be confused to find them among macro-crystalline varieties. To understand the difference between the two groups it is best to look at the way the different varieties form in nature.

In addition to these varieties sometimes form varieties or growth forms are distinguished. These are defined by their general shape and are given names that reflect their resemblance with another object, like scepter quartz, artichoke quartz or needle quartz. Note that these categories are only used for macro-crystalline varieties. If you're looking for a specific variety and don't find it among the links in the left menu, check the Table of Synonyms to see if it is just a synonym of some variety.

Occurrence

Quartz is very abundant. It can be found in many different geological environments, and its visual appearance reflects the various conditions under which it was formed. Large parts of the earth's surface are literally covered with quartz - sand left over from the weathering of rocks, due to its great physical and chemical resistance. Along with calcite, quartz is one of the few minerals that form rocks made up almost entirely of them: quarzites and sandstones. The greatest amounts are hidden in granites and related rocks, though, which contain about 5-50% quartz. The earth's crust as a whole contains about 12% quartz, most of it in continental crust. The earth's mantle and core are completely void of quartz, and very likely the upper mantle does not contain any free silica at all.

Use

Colored quartz varieties have been used for jewelery for ages, but most quartz is used as a component of concrete: quartz sand and quartz gravel. Pure quartz is needed for producing glass, ceramics, and chemical apparatus. Quartz glass, also known as "fused quartz" or "fused silica" (produced by quickly cooling molten quartz) has a number of interesting properties: its thermal expansion coefficient is very low, it is transparent for ultraviolet light, it is chemically almost inert, and it can form very thin but strong threads used in physical instruments.

A well known application of quartz is its use as an oscillator in electric circuits in watches and computers. Less well known is, for example, its use as a membrane in ultrasonic devices. Quartz is of course the major "ore" of silicon, used in the integrated circuits ("chips") of your computer.

Last but not least, the on-going New Age movement and its associated esoteric culture has (re-)discovered quartz, and probably most of the quartz web pages in the Internet are related to the marketing of rock crystals and other quartz varieties for esoteric purposes. Because of its technical importance, its abundance, and its role in geologic processes, quartz is probably the best-studied of all minerals. But still not everything about it is understood.


http://www.quartzpage.de/intro.html