tourmaline boron silicate minerals

Tourmaline

Tourmaline is a birthstone for October, along with opal. “Tourmaline” is the name of a large group of boron silicate minerals. These minerals share a common crystal structure (Trigonal) and similar physical properties but vary tremendously in chemical composition. They share the elements silicon, aluminum, and boron, but contain a complex mixture of other elements such as sodium, lithium, calcium, magnesium, manganese, iron, chromium, vanadium, fluorine, and sometimes copper. Tourmaline is
perhaps the main repository for boron in the earth’s crust. Hardness is 7-7.5 on the Mohs Scale.

Tourmaline – Boron silicate minerals

Tourmaline is the world’s most colorful gemstone. A tourmaline’s chemical composition directly influences its physical properties and is responsible for its color. However, color is a poor method to correctly identify a tourmaline. You can’t just look at the color and know what it is.

Zoning in tourmalines is common, not only from top to bottom but also from the core outward. Some of this zoning is obvious in transparent and translucent crystals and is one of the reasons why collectors are so attracted to them. The more vivid the color change and the more colors a tourmaline crystal has, the more desirable it is to collectors. Opaque tourmalines, commonly the black and brown ones, also can have significant compositional zoning. Tourmaline has a prismatic crystal habit and often has obvious striations that parallel the long axis of a crystal. Tourmaline crystals often have triangular or six-sided cross-sections with rounded edges.

Varieties of Tourmaline

Likewise, there are 38 minerals in the tourmaline group of boron silicate minerals recognized by the International Mineralogical Association based upon the chemical composition of solid solution series end members. The major tourmaline species are elbaite, liddicoatite, dravite, uvite, and schorl.

Elbaite is a sodium lithium aluminum tourmaline species. Most gem-quality tourmalines are of the Elbaite species. As a result, Elbaites offer the widest range of gem-quality tourmaline colors. They can be green, blue, or yellow, pink to red, colorless, or zoned with a combination of colors.

Liddicoatite is a calcium lithium tourmaline species. Colors include light-brown, pink to red, green, and blue.

Dravite is a sodium magnesium-aluminum tourmaline species. Colors include pale brown to dark-brown to brownish-black, also dark-yellow, blue.

Schorl is a sodium iron aluminum tourmaline species. Colors include bluish-black to black, sometimes brownish-black.

Uvite is a calcium magnesium aluminum tourmaline species. As a result, the colors are mostly deep green or brown.

Uvite Tourmaline boron silicate minerals

Occurrence

Tourmalines are found in many geological settings all over the world. Pegmatites produce most of the crystal specimens cherished by collectors but tourmalines are also commonly found in granites and in metamorphic rocks like schist, shale, marble, conglomerate, lodestone, and limestone. They are also found in sedimentary rocks because of their hardness and resistance to chemical weathering.

Brazil has been the world’s leading source of tourmaline for nearly 500 years. Since the late 1800’s, millions of carats of tourmaline have been produced from the pegmatite deposits of Minas Gerais, Brazil.

In the United States, significant amounts of pink and green tourmaline have been produced from dozens of Maine localities. The most important source of gem tourmaline in the United States has been the tourmaline mines of southern California, primarily in Riverside and San Diego Counties.

However, In Colorado, three tourmaline species are known to occur: schorl, dravite, and elbaite. Schorl is always black and forms elongated, striated crystals. Elbaite is usually dark green, blue, or pink. Dravite is brown. The best localities are in Clear Creek, El Paso, Fremont, Gunnison, and Lake counties.

Recent discoveries of tourmaline of various kinds are made in Afghanistan, Mozambique, Namibia, Nigeria, Pakistan, Tanzania, and other countries.

Trade Names

Metaphysical Uses

(Book of Stones from Simmons and Ahsian)

Rubellite tourmaline boron silicate minerals

Tourmaline boron silicate minerals Article by Bill Jones, Sidewinder Minerals.

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