Tourmaline

The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. Its composition varies widely because of isomorphous replacement (solid solution), and its general formula can be written as XY3Z6(T6O18)(BO3)3V3W, where:[3]

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X = Ca, Na, K, vacancy tuomalin_com.cn

Y = Li, Mg, Fe2+, Mn2+, Zn, Al, Cr3+, V3+, Fe3+, Ti4+

Z = Mg, Al, Fe2+, Fe3+, Cr3+, V3+ 中国托_玛琳网www_tuomalin_com_cn

T = Si, Al, B

B = B, vacancy 转载请注明来源：中国托玛琳网 www.tuomalin.com.cn

V = OH, O

W = OH, F, O 中国托玛琳网为您专业讲解托玛琳（Tourmaline）知识，提供托玛琳产品贸易平台.让中国人更了解托玛琳！

Tourmaline belongs to the trigonal crystal system and occurs as long, slender to thick prismatic and columnar crystals that are usually triangular in cross-section. Interestingly, the style of termination at the ends of crystals is asymmetrical, called hemimorphism. Small slender prismatic crystals are common in a fine-grained granite called aplite, often forming radial daisy-like patterns. Tourmaline is distinguished by its three-sided prisms; no other common mineral has three sides. Prisms faces often have heavy vertical striations that produce a rounded triangular effect. Tourmaline is rarely perfectly euhedral. An exception was the fine dravite tourmalines of Yinnietharra, in western Australia. The deposit was discovered in the 1970s, but is now exhausted. Tourmaline www.tuomalin.com.cn

All hemimorphic crystals are piezoelectric, and are often pyroelectric as well. Tourmaline crystals when warmed become positively charged at one end and negatively charged at the other. Due to this effect, tourmaline crystals in collections may attract unsightly coatings of dust when displayed under hot spotlights. Tourmaline's unusual electrical properties made it famous in the early 18th century. Brightly colored Sri Lankan gem tourmalines were brought to Europe in great quantities by the Dutch East India Company to satisfy a demand for curiosities and gems. At the time it was not realised that schorl and tourmaline were the same mineral. zhong-guo-tuo-ma=lin-wang www.tuomalin.com.cn

Tourmaline has a wide variety of colors. Usually, iron-rich tourmalines are black to bluish-black to deep brown, while magnesium-rich varieties are brown to yellow, and lithium-rich tourmalines are practically any color: blue, green, red, yellow, pink etc. Rarely, it is colorless. Bi-colored and multicolored crystals are relatively common, reflecting variations of fluid chemistry during crystallisation. Crystals may be green at one end and pink at the other, or green on the outside and pink inside: this type is called watermelon tourmaline. Some forms of tourmaline are dichroic, in that they appear to change color when viewed from different directions.

The most common variety of tourmaline is schorl. It may account for 95% or more of all tourmaline in nature. The early history of the mineral schorl shows that the name "Schorl" was in use prior to the year 1400 (AD) because a village known today as Zschorlau (in Saxony, Germany) was then named "Schorl" (or minor variants of this name). This village had a nearby tin mine where, in addition to cassiterite, a lot of black tourmaline was found. The first relatively detailed description of schorl with the name "schürl" and its occurrence (various tin mines in the Saxony Ore Mountains) was written by Johannes Mathesius (1504-1565) in 1562 under the title "Sarepta oder Bergpostill" (Ertl, 2006). Up to about 1600, additional names used in the German language were "Schurel", "Schörle", and "Schurl". From the 18th century on, the name "Schörl" was mainly used in the German-speaking area. In English, the names "shorl" and "shirl" were used in the 18th century for schorl. In the 19th century the names "common schorl", "schörl", "schorl" and "iron tourmaline" were used in the Anglo-Saxon area (Ertl, 2006). The word tourmaline has two entymologies, both from the Sinhalese word turamali, meaning "stone attracting ash" (a reference to its pyroelectric properties) or according to other sources "mixed gemstones". The meaning of the word "schorl" is a mystery, but it may be a Scandinavian word.

The name dravite was used for the first time by Gustav Tschermak (*1836 - †1927; Professor of mineralogy and petrography at the University of Vienna) in his book “Lehrbuch der Mineralogie” (published in 1884) for Mg-rich (and Na-rich) tourmaline from the village Unterdrauburg, Drava river area, Carinthia, Austro-Hungarian Empire. Today this tourmaline locality (type locality for dravite) at the village Dravograd (near Dobrova pri Dravogradu), is a part of the Republic of Slovenia (Ertl, 2007). Tschermak gave this tourmaline the name dravite, for the Drava river area, which is the district along the Drava River (in German: Drau, in Latin: Drave) in Austria and Slovenia. The chemical composition which was given by Tschermak in 1884 for this dravite approximately corresponds to the formula NaMg3(Al,Mg)6B3Si6O27(OH), which is in good agreement (except for the OH content) with the endmember formula of dravite as known today (Ertl, 2007).