Tourmaline
The mineral tourmaline is a complex silicate of aluminium and boron, but because of isomorphous replacements (solid solution) it varies widely in composition with iron, magnesium and lithium also entering into the combination to a greater or lesser extent. Tourmaline belongs to the hexagonal crystal system, and oftentimes occurs as long, slender, prismatic acicular crystals that are usually terminated with three faces of a rhombohedron. The smaller crystals are often found in a radial pattern, and collumnar masses are common. The prisms are usually three, six, or nine sided with heavy vertical striations producing a rounded effect. Tourmaline is essentially without cleavage, its fracture is conchoidal to even, its hardness is 7-7.5, its specific gravity is 2.9-3.2, and it has a vitreous lustre inclining to resinous.
Tourmaline has a wide variety of colors. Common tourmaline can be black, bluish-black, brown, blue, green, red or pink; transparent varieties can be colourless (rare), various shades of rose or pink, greens, blues and brown. Bi-colored crystals are common and can be green at one end and pink at the other, or green on the outside and pink within, which is very attractive in the case of transparent tourmalines.
The opaque black tourmalines were originally called schorl, a term which was applied to all tourmalines until 1703 when the word tourmaline was introduced as a corruption of the Ceylonese word turamali. The origin of the word schorl is not known but could be Scandanavian.
Other names for tourmalines:
- Dravite subgroup:
- Brown - dravite (from the Drave district of Carinthia)
- Schorl subgroup:
- Elbaite subgroup:
- Rose or pink - rubellite (from ruby)
- Dark blue - indicolite (from indigo)
- Light blue - Brazillian sapphire
- Green - verdelite or Brazillian emerald
- Colorless - achroite (from the Greek for "colorless")
Small tourmalines are found in granite and some gneisses. Due to the mineralizing action of magmatic vapors, tourmaline is found particularly well-developed in pegmatites and as a contact metamorphorphic mineral.
See also: List of minerals
Referenced By
Crystal habit | Gemstone | Jewel | List of chemistry topics | List of minerals
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