Tourmaline
From Wikinfo
The mineral tourmaline is chemically one of the most complicated silicate minerals, it is a complex silicate of aluminium and boron, but because of isomorphous replacement (solid solution) its composition varies widely with iron, magnesium and lithium and other elements entering into the combination to a greater or lesser extent.
Tourmaline belongs to the trigonal crystal system and often occurs as long, slender to thick prismatic and columnar crystals that are usually terminated by three triangular faces of a rhombohedron. Interestingly the style of termination at the ends of crystals is asymmetrical, a property is known as hemimorphism. Smaller crystals in fine grained granite called aplite may form radial daisy like patterns. Prisms are usually three sided and often with heavy vertical striations that produce a rounded triangular effect. Tourmaline is rarely truly euhedral, an exception are the Dravite tourmalines from Yinnietharra, Australia.
All hemimorphic minerals are piezoelectric and pyroelectric, tourmaline is no exception. When warmed, tourmaline crystals will become positively charged at one end and negatively charged at the other, somewhat like a battery. Some tourmaline crystals in museum collections attract an unsightly coating of dust when displayed under hot spotlights. It was this unusual property of tourmaline that made it famous in the early 18th century, when in particular Sri Lankan tourmaline was brought to Europe in great quantities by the Dutch east India Company.
Tourmaline has a good to poor prismatic and poor rhombohedral cleavage. Its fracture is subconchoidal to even, it has Mohs hardness of 7-7.5. Its specific gravity is 2.9-3.2, higher in iron bearing varieties. It is transparent to opaque with a vitreous lustre inclining to resinous in dark specimens.
Tourmaline has a wide variety of colors. Usually it is iron rich and black, bluish-black to deep brown, magnesium rich varieties are brown to yellow, and lithium rich tourmalines maybe blue, green, red, yellow or pink; most rarely of all tourmaline is colourless. Bi-colored and multicoloured crystals are relatively common, reflecting variations of fluid chemistry during crystalisation, crystals maybe green at one end and pink at the other or green on the outside and pink within, watermelon tourmaline.
Common opaque black tourmaline was originally called schorl and was first described by Mathesius in 1524. The term schorl was applied to all tourmalines until 1703 when the word tourmaline was introduced as a corruption of the Ceylonese word turamali meaning stone attracting ash. The origin of the word schorl is not known but could be Scandanavian.
Tourmaline is found in two main geological occurrences. Igneous rocks, in particular granite and granite pegmatite and in metamorphic rocks such as schist and marble. Schorl and lithium rich tourmalines are usually found in granite and granite pegmatite. Mg rich tourmalines, dravites, are generally restricted to schist and marble. Also, tourmaline is a durable mineral and can be found in minor amounts as grains in sandstone and conglomerate.
Tourmaline is used in jewellery, pressure gauges and specialist microphones. In jewellery, blue indicolite is the most expensive, followed by verdelite and rubellite. Ironically the rarest variety, achroite, is not appreciated and it the least expensive of the gem tourmalines.
Other names for tourmalines:
- Dravite subgroup:
- Brown - dravite (from the Drave district of Carinthia)
- Schorl subgroup:
- Black - schorl
- Elbaite subgroup: named after the island of Elba, Italy
See also: List of minerals
References
- Adapted from the Wikipedia article, "Tourmaline" http://en.wikipedia.org/wiki/Tourmaline, used under the GNU Free Documentation License
