Tourmaline
From Freepedia
| Tourmaline | |
|---|---|
| General | |
| Category | Mineral Group |
| General Chemical formula | Na(Al,Fe,Li,Mg,Mn)M3Al(Si6O18)(BO3)3(OH,F)4 |
| Identification | |
| Color | Most commonly black, but can range from brown, violet, green, pink, or in a dual-colored pink and green. |
| Crystal habit | Parallel and elongated. Acircular prisms, sometimes radiating. Massive. Scattered grains (in granite). |
| Crystal system | Trigonal |
| Cleavage | Good to poor prismatic. Poor rhombohedral |
| Fracture | Subconchoidal to even |
| Mohs Scale hardness | 7 - 7.5 |
| Luster | Vitreous, sometimes resinous |
| Refractive index | nω=1.635 - 1.675 nε=1.610 - 1.650 |
| Pleochroism | None |
| Streak | Colorless |
| Specific gravity | 3.02 - 3.26 |
| The 14 recognized minerals in the group (endmember formulas) | |
| Elbaite | Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)4 |
| Schorl | NaFe2+3Al6Si6O18(BO3)3(OH)4 |
| Dravite | NaMg3Al6Si6O18(BO3)3(OH)4 |
| Chromdravite | NaMg3Cr6Si6O18(BO3)3(OH)4 |
| Olenite | NaAl3Al6Si6O18(BO3)3O3OH |
| Buergerite | NaFe3+3Al6Si6O18(BO3)3O3F |
| Povondraite | NaFe3+3(Fe3+4Mg2Si6O18(BO3)3(OH)3O |
| Vanadiumdravite | NaMg3V6Si6O18(BO3)3(OH)4 |
| Liddicoatite | Ca(Li2Al)Al6Si6O18(BO3)3(OH)3F |
| Uvite | CaMg3(MgAl5Si6O18(BO3)3(OH)3F |
| Hydroxy-feruvite | CaFe2+3(MgAl5Si6O18(BO3)3(OH)4 |
| Rossmanite | (LiAl2)Al6Si6O18(BO3)3(OH)4 |
| Foitite | (Fe2+2Al)Al6Si6O18(BO3)3(OH)4 |
| Magnesiofoitite | (Mg2Al)Al6Si6O18(BO3)3(OH)4 |
The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. 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.
The most common variety of tourmaline is schorl,first described by Mathesius in 1524. It may account for 95% or more of all tourmaline in nature. The word tourmaline is a corruption of the Sinhalese word turamali, meaning "stone attracting ash" (a reference to its pyroelectric properties). The meaning of the word "schorl" is a mystery, but it may be a Scandinavian word.
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. Magnesium-rich tourmalines, dravites, are generally restricted to schists 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 jewelry, pressure gauges, and specialist microphones. In jewellery, blue indicolite is the most expensive, followed by green verdelite and pink rubellite. Ironically the rarest variety, colourless achroite, is not appreciated and is the least expensive of the transparent 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
References
- Tourmaline classification Accessed 9/12/2005
- Mindat tourmaline group Accessed 9/12/2005
- Mineral galleries Accessed 9/12/2005
