General Chemical formula of Tourmaline Mineral Group is Na(Al,Fe,Li,Mg,Mn)M₃Al(Si₆O₁₈)(BO₃)₃(OH,F)₄. Its Most commonly Color is black, but can range from brown, violet, green, pink, or in a dual-colored pink and green.

Some of its features goes as follows:
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    n��=1.635 - 1.675 n��=1.610 - 1.650
Pleochroism None
Streak Colorless
Specific gravity 3.02 - 3.26

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.

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.

Formation of Tourmaline:
Tourmaline occurs as an accessory mineral in five of six zones in the Li-, B-, Be-, Nb-, Ta-, and Sn-enriched, internally zoned Bob Ingersoll No.1 pegmatite located in the Precambrian core of the southern Black Hills near Keystone, South Dakota. The purpose of this investigation is
(1) to examine the usefulness of tourmaline as a petrogenetic indicator in this setting and
(2) to apply this indicator to interpret the internal evolution of the pegmatite. Tourmaline occurs in ten distinct types based on texture or habit and coexisting mineral assemblage. Chemical analyses of the tourmaline show compositional differences between types that may be useful in determining the crystallization sequence and differentiation mechanisms. Trends of maj or-element variations in tourmaline from the country rock to the core include the following:
(1) Mg and Ti decrease abruptly from the country rock through the border zone to the wall zone;
(2) Fe decreases and (Li + AI) increase from the wall zone to the core; and the minor elements Mn, Zn, and Ca generally increase toward the core. Tourmaline compositional trends in this pegmatite, coupled with textural features and mineral associations, provide evidence of an inward, generally sequential crystallization of the border zone, wall zone, intermediate zones, and finally the core, but with overlap between the inner wall zone and intermediate zones. The pegmatite melt was probably saturated at the onset of crystallization; however, fluid exsolution may have temporarily stopped during the initial crystallization of the third intermediate zone and a period of tourmaline instability, but then resumed, along with tourmaline crystallization, during the formation of the pegmatite core.

Tourmaline is commonly the coarsest mineral in the border zone, attaining lengths up to 2.5 cm. Tourmaline occurs as slender, tapered crystals that are oriented sub-perpendicular to the contact with country rock, with the tapered end nearest to the contact. Some of the crystals are inclined or nearly parallel to the contact, a feature that may represent flow alignment or may indicate the presence of inclined gradients caused by the flowage of melt interior to the border zone as it crystallized. The border zone is nearly continuous around the pegmatite and ranges in thickness from I to 10 cm. The transition from border to wall zone is sharp, commonly occurring over a distance on the order of entimeters, and is marked by an abrupt increase in grain size.

Common mineral variation is observed at different places within the same body of pegmatite. In these pegmatites, muscovite, quartz, tourmalineand feldspar seem to have developed mainly in two distinct generations, for example, the muscovite of earlier generation forms tiny "books" while later crystals occur as small disseminated flakes in the groundmass of less coarser quartz. The earlier quartz is strongly deformed, whilethe later occurs as veins of variable dimensions. The black tourmaline of earlier generation is represented by schorlite and of later origin hasbeen identified as dravite. It may be concluded that these pegmatites were formed by the influx of fluids rich in silica, Al and K that filled thefractures and other planes of weakness (foliation) in the host rocks. The alkali components while migrating towards the wall of the country rocks, crystallised as muscovite and feldspar. In larger pegmatite bodies of Suradevi Hills, the development and concentration of muscovite along theircontact with the mica schist is probably due to the migration of Al and K and volatiles towards the border of the intrusion. This crystallizationof muscovite in preference to potash-feldspar may be due to the presence of high contents of volatiles.

However, common mineral variation is observed at different places within the same body of pegmatite. In these pegmatites, muscovite, quartz, tourmalineand feldspar seem to have developed mainly in two distinct generations, for example, the muscovite of earlier generation forms tiny "books" while later crystals occur as small disseminated flakes in the groundmass of less coarser quartz. The earlier quartz is strongly deformed, whilethe later occurs as veins of variable dimensions. The black tourmaline of earlier generation is represented by schorlite and of later origin hasbeen identified as dravite. It may be concluded that these pegmatites were formed by the influx of fluids rich in silica, Al and K that filled thefractures and other planes of weakness (foliation) in the host rocks. The alkali components while migrating towards the wall of the country rocks, crystallised as muscovite and feldspar. In larger pegmatite bodies of Suradevi Hills, the development and concentration of muscovite along theircontact with the mica schist is probably due to the migration of Al and K and volatiles towards the border of the intrusion. This crystallizationof muscovite in preference to potash-feldspar may be due to the presence of high contents of volatiles.

From Wikipedia, "Tourmaline as a recorder of pegmatite evolution" from BRADLEY L. JOLLIFF, JAMES J. PAPIKE, CHARLES K. SHEARER, and "GRANITIC PEGMATITES OF KORADI-KOLAR SECTOR"

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