Tourmaline

The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. more...

It is a complex silicate of aluminium and boron, but because of isomorphous replacement (solid solution), its composition varies widely with sodium, calcium, iron, magnesium, lithium and other elements entering into the structure.

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.

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 coloured Sri Lankan gem tourmalines were brought to Europe in great quantities by the Dutch East India Company to satisfy demand as curios and gems. At the time it was not realised that schorl and tourmaline were the same mineral.

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 colourless. Bi-colored and multicoloured crystals are relatively common, reflecting variations of fluid chemistry during crystalisation. 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.

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