TOURMALINE MINERAL FACTS Nevada Turquoise gem stones
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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 s
trongly 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
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:

1. Alkali tourmalines, which are colorless, red or green, and transparent. This variety is  known as Elbaite and it comprises most of the gem varieties.
2. Iron tourmalines, which are usually dark blue or black and translucent. This is a common gangue mineral known as Schorl.
3. Magnesium tourmalines, which are yellowish brown, or brownish and translucent. It does produce a few gem varieties, especially in the color orange.
4. Chrome tourmalines, which are dark green, black and translucent,or colorless and transparent. This is the rarest of the four groups, although this type does produce some emerald green gems.

Tourmaline forms handsome crystals that are frequently characterized by possessing vertical striations and a triangular cross-section. They crystallize in the rhombohedral division of the hexagonal system and are hemimorphic (ditrigonal pyramidal class). The crystals are usually prismatic or columnar in habit, and are terminated by rhombohedrons.  The name tourmaline comes from turamali, a name given to the early parcels of mixed gemstones from Ceylon.

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.

Pink Tourmaline
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.

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 Spray, Africa

Tourmaline Crystal, Mesa Grande
Tourmaline Crystal, Mesa Grande

Tourmaline Crystal, Maine, USA

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.

It is found also as an accessory mineral in metamorphic rocks, such as gneisses, schists and crystalline limestones.  The black form of tourmaline is of widespread occurrence as an accessory mineral in metamorphic rocks. It may also occur as a gangue mineral associated with various types of ore bearing veins including copper, lead and cobalt minerals. In these metal deposits, the presence of tourmaline usually indicates formation at fairly high temperatures.

Tourmaline is so common that an enumeration of its occurrence locations is impossible in a short article. The light colored, lithium bearing gem varieties are found in the pegmatite dikes associated with lepidolite. Famous woorld wide localities for the occurrence of the gem tourmalines are the island of Elba; at Campolonga, Switzerland; and Penig, Saxony, in the state of Minas Geraes, Brazil; in the Ural Mountains near Ekaterinburg; at several locations on the island of Madagascar. In recent years important finds have been made in Africa, including Tanzania and Nigeria. Most of these are found as deposits in pegmatite dikes, but in some locations, including Ceylon, Brazil and Madagascar, water rounded tourmaline has been recovered from detrital deposits formed from erosion of the dikes as well.

In the US, gem tourmaline occurs at several points in western and central Maine; at Haddam, Conn.; and in San Diego Co., in California. The Maine localities are at Hebron, Paris, Poland and Auburn. The tourmalines are in pockets in pegmatites. The green varieties are most common, but all colors occur, and many crystals are variegated. It has also been mined at Haddam Neck, Connecticut. The centers of the gem industry in California are Mesa Grande, Pala, Rincon and Ramona in San Diego County, where many pink tourmalines and a few green crystals occur associated with the lithium mica, lepidolite, in pockets in a number of  pegmatite dikes.

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
Tourmaline on quartz, San Diego County, California

Multiple Colors of Tourmaline Crystals

Multiple Colors of Tourmaline Crystals

Tourmaline Queen specimen, Pala, California

Tourmaline Queen specimen, Pala, California

Tourmaline with Quartz and Feldspar, Calif.

    Tourmaline with Quartz and Feldspar, Calif.

Above: Black Schorl Tourmaline on Feldspar, Brazil
Above: Black Schorl Tourmaline on Feldspar, Brazil

Above: Green Tourmaline on Feldspar, Brazil
Above: Green Tourmaline on Feldspar, Brazil



Please note that the author, Chris Ralph, retains all copyrights to this entire document and it may not be reproduced, quoted or copied without permission.

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