Ch. 5: Tourmaline in Maine Pegmatites

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	TOURMALINE. 143

	In most years the importations of mica into the United States are largely in excess of the domestic production in value. They come mainly from India and Canada. TOURMALINE. Chemical and physical properties.—Tourmaline is a complex silicate of boron and aluminum containing various amounts of either magnesium, iron, or the alkali metals. The form of the more perfect crystals is commonly that of a three-sided prism, the sides of the prism usually being striated and channeled (Pl. XV). In many crystals the three-sided form is somewhat modified by the combination with it of a hexagonal prism. The latter is usually subordinate and has the effect of merely somewhat rounding the angles of the triangular prism. Many crystals are terminated by three planes forming a low pyramid, but in others the number of terminal planes is very large. The hardness (7 to 7.5) is slightly greater than that of quartz. There is no well-defined cleavage. The mineral exhibits a great variety of colors, ranging from black through brownish-black and blue-black to blue, green, red, pink, and colorless. The red varieties go under the name of rubellite; the blue varieties are known as indicolite and the colorless as achroite. A crystal may be green at one end and red at the other or in cross section may show a blue center, then a zone of red, and then one of green. Some of the crystals from Paris, Oxford County, grade from white at one termination to emerald green, then light green, then pink, and finally are colorless at the other termination. The color is dependent on the chemical composition, the green, blue, pink, and colorless varieties generally being rich in lithium and manganese and the dark opaque varieties being particularly rich in iron. The color in the transparent varieties varies with the direction in which the light penetrates the gem; thus a crystal which, when viewed from the side, is a transparent green, may be opaque or yellow-green when viewed along the length of the prism. Because of this property of dichroism, as it is called, it is usually necessary in cutting gem tourmalines to make the "table" of the stone parallel to the long axis of the crystal. Another distinctive quality of the mineral is that it becomes electrified when warmed slightly and is then capable of picking up ashes, small scraps of paper, etc. Occurrence.—Tourmaline occurs in small crystals in a great variety of rocks and may be either an original crystallization or the result of metamorphic processes. Large crystals and those which are of gem value occur only in the pegmatite deposits. The black varieties occur almost exclusively in the solid pegmatite associated with quartz and feldspar and without any regularity in arrangement. The black varieties may contain from 3 to nearly 20 per cent of oxides of iron

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