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Tourmaline Mineral Facts:

Chemical Formula: (R₉ Al₃ (B-OH-F)₂ Si₄O₁₉), where R is a combination of: H, Al, Mg, Fe, Al, Cr, Fe, K, Na
The mineral is strongly pyroelectric; i.e., when cooling from being heated to about 100 C. it develops positive electricity at one end of the crystal and negative at the other, which enables the crystal to attract and hold bits of paper, etc.

Colors: The color is varied, depending upon the composition. Common tourmaline with much iron is black, sometimes brown. More rarely light colored in fine shades of red, pink, green, blue, yellow, etc. Rarely white or colorless. A single crystal may show several different colors either arranged in concentric bands about the center of the crystal or in transverse layers along its length.

Hardness: 7 to 7.5

Density: 2.98 to 3.2

Cleavage: None  

Crystallography: Hexagonal
Usually in crystals. Sometimes massive compact; also coarse to fine columnar, either radiating or parallel. Crystals usually prismatic, vertically striated. A triangular prism, with three faces, prominent, which with the tendency of the prism faces to be vertically striated and to round into each other gives the crystals usually a cross section like a rounded triangle. Crystals are commonly terminated by base and low positive and negative rhombohedrons ; sometimes scalenohedrons are present. When the crystals are doubly terminated they usually show different forms at the opposite ends of the vertical axis.

Luster:. Its luster is vitreous.

Optics: (Refractive Index):  w = 1.6422; e = 1.6225;
The above is the refractive indices for yellow light in colorless crystals. In the iron-bearing schorl varieties the refraction is stronger.

Bi-color Tourmaline Crystal 

Composition, Structure and Associated Minerals:
Tourmaline is a complex silicate of boron and aluminum, containing varying amounts of ferrous iron, magnesium, manganese, calcium, sodium, potassium, lithium, hydroxyl and fluorine. It is of great scientific interest because of its complex crystallization, its handsome crystals and the physical properties which it exhibits so beautifully. Moreover, it furnishes gems of many colors, which, because of their brilliancy, are greatly admired by many persons. The mineral appears to be a derivative of the alumino-borosilicic acid  H₉Al₃(B-OH)₂Si₄O₁₉ in which the hydrogen may be replaced by Al, by Cr, by Mg and Fe or by Li or Na, giving rise to four groups of compounds between which are many gradations. Moreover, in most specimens a portion of the hydroxyl is replaced by fluorine. In other words the mineral is an isomorphous mixture of several substances that are derivatives of the alumino-borosilicic acid mentioned. The four groups of tourmalines that are clearly distinguishable are:

Tourmaline also forms one of the most beautiful of the semiprecious gem stones. The color of the stones varies, the principal shades being olive-green, pink to red and blue. Sometimes a stone is so cut as to show different colors in different parts. The green-colored stones are usually known by the mineral name, tourmaline, or as Brazilian emeralds. The red or pink stones are known as rubellite, while the rarer dark blue stones are called indicolite.

Above: Pink Tourmaline

Identification and Diagnostics
Tourmaline is readily distinguished from all other minerals by its crystallization (the characteristic rounded triangular cross section of the crystals), its hardness, lack of cleavage and a positive reaction for the presence of boron. In massive forms it differs from garnet and vesuvianite which it somewhat resembles by its difficult fusibility and brittleness. The mineral is, on the whole, very stable. It is known, however, to alter into mica, chlorite and steatite. Tourmaline also exhibits very strong pleochroism especially the darker varieties. Viewed in the direction of the c axis the mineral is always, except in the case of colorless varieties, darker than when viewed in a direction at right angles to it.

Uses:
The transparent varieties are used principally as gem stones, and the darker, translucent varieties in optical instruments. Fine crystal forms are valued as mineral specimens. As a gemstone, Tourmaline has gained a wide acceptance and become much more important in recent decades.

Tourmaline Crystal Spray, Africa

Tourmaline Crystal, Mesa Grande

Above: Tourmaline Crystal, Maine, USA

Occurrence, Localities and Origins:
Tourmaline occurs as an accessory mineral in many granites, syenites, and the more acid igneous rocks generally.  It occurs in pegmatites, in quartz and ore veins, and in limestones and schists on the peripheries of granite masses where it is the result of contact action. It occurs also as an original, pyrogenic mineral in acid igneous rocks. The variety in limestone is usually magnesium rich and brown colored. It is one of the most common and characteristic minerals formed by circulating hot waters. That is, it is a mineral that has been formed at high temperatures and pressures through the agency of liquids carrying boron, fluorine, etc. It is found, therefore, commonly as an accessory mineral in pegmatite veins, or dikes, occurring with granite intrusions. It is associated with the ordinary minerals of granite pegmatite, orthoclase, albite, quartz and muscovite; but also with some less common ones such as lepidolite, beryl, apatite, fluorite, etc.

Also in the United States, fine brown tourmaline crystals occur in the limestone at Gouverneur, N. Y. ; and handsome black ones at Pierrepont, N. Y. ; New Hope, Penn. ; and in Alexander Co., N. C.

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Tourmaline on quartz, San Diego County, California
 

Multiple Colors of Tourmaline Crystals

Tourmaline Queen specimen, Pala, California

    Tourmaline with Quartz and Feldspar, Calif.

Above: Black Schorl Tourmaline on Feldspar, Brazil

Above: Green Tourmaline on Feldspar, Brazil

Nevada Outback Gems