Tourmaline Composition, Crystallization & Structure
Composition. A complex silicate of boron and aluminum, containing varying amounts of ferrous iron, magnesium, manganese, calcium, sodium, potassium, lithium, hydroxyl and fluorine.
Crystallization. Hexagonal-rhombohedral; hemimorphic. 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 sphericaltriangle. 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 (homomorphism).
Structure. Usually in crystals. Sometimes massive compact; also coarse to fine columnar, either radiating or parallel.
Physical Properties. Vitreous to resinous luster. Color 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. 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, ete. Strongly diachronic; Le., light traversing the crystal in one direction may be of quite a different color or shade of color from that traversing the crystal in a direction at right angles to the first. H. = 7-7.5; G. = 2.98-3.2.
Tests. To be recognized usually by the characteristic rounded triangular cross section of the crystals; absence of prismatic cleavage, coal-like fracture of black variety.
Occurrence. Tourmalinc is one of the most common and characteristic minerals formed by pneumatolytic action. That is, it is a mineral that has been formed at high temperatures and pressures through the agency of vapors carrying boron, fluorine, ete. It is found, there fore, commonly as an accessory mineral in pegmatite veins, 01' dikes, occurring with granite intrusions. Associated with the ordinary minerals of granite pegmatite, orthoclase, albite, quartz and muscovite; also with lepidolite, beryl, apatite, fluorite, ete.
Found also as an accessory mineral in metamorphie roeks, such as gneisses, schists and crystalline limestones.
The black tourmaline is of widespread occurrence as an accessory mineral in metamorphie rock. The light colored gem varieties are found in the pegmatite dikes. Famous localities for the occurrence of the gem tourmalines are the island of Elba; in the state of Minas Geraes, Brazil; Ural Mountains near Ekaterinburg; Madagasear; Paris and Auburn, Maine Chesterfield, Massachusetts; Haddam Neck, Connecticut; Mesa Grande, Pala, Rincon and Ramona in San Diego County, California. Brown crystals are found near Gouverneur, New York and fine black crystals at Pierrepont, New York.
Name. The name turmoline comes from turamali, a name. Given to the early gems from Ceylon.
Use. Tourmaline forms one of the most beautiful of the semiprecious gem stones. The color of the stone’s 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.
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