Tourmaline

Did You Know?

• Tourmaline was sometimes called the "Ceylonese Magnet" because it could attract and then repel hot ashes due to its pyroelectric properties.
• Chemists in the 19th century used tourmalines to polarize light by shining rays onto a cut and polished surface of the gem.
• Tourmaline is part of the ZTR index for highly weathered sediments.

Species and varieties

Commonly encountered species and varieties of tourmaline include schorl, dravite, and elbaite. Schorl, the sodium iron endmember, is the most common species, possibly accounting for 95% or more of all tourmaline in nature. The name "schorl" was in use before 1400; a village now called Zschorlau in Saxony, Germany, was then named "Schorl" and had a tin mine where black tourmaline occurred. Johannes Mathesius first described schorl as "schürl" in 1562. Dravite, also called brown tourmaline, is the sodium magnesium endmember and is named after the Drave district of Carinthia. Uvite is a calcium magnesium tourmaline. Elbaite, named after the island of Elba, Italy, includes the red or pinkish-red rubellite variety, the light blue to bluish-green indicolite variety, the green verdelite variety, and the colorless achroite variety.

Chemical composition

The tourmaline mineral group is chemically one of the most complicated silicate mineral groups, with composition varying widely due to isomorphous replacement. Its general formula can be written as XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W, where X can be calcium, sodium, potassium, or a vacancy; Y includes lithium, magnesium, iron, manganese, zinc, aluminum, chromium, vanadium, titanium, or a vacancy; Z includes magnesium, aluminum, iron, chromium, or vanadium; T is silicon, aluminum, or boron; B is boron or a vacancy; V is hydroxyl or oxygen; and W is hydroxyl, fluorine, or oxygen. A revised nomenclature for the group was published in 2011.

Physical properties

Tourmaline belongs to the cyclosilicate family, with a six-member ring structure and a trigonal crystal system. Its crystals are typically long and prismatic, ranging from slender to thick, and often show a triangular cross-section with curved, striated faces. The ends of these crystals can be asymmetrical, a trait known as hemimorphism. In fine-grained granite called aplite, small slender prisms frequently form radial patterns resembling daisies.

What sets tourmaline apart from other common minerals is its three-sided prisms, which often feature heavy vertical striations that create a rounded triangular appearance. Perfectly formed crystals are rare; one notable exception was the dravite tourmaline from Yinnietharra in western Australia, found in the 1970s but now depleted. All hemimorphic tourmaline crystals are piezoelectric and often pyroelectric. The crystal structure is built from six-member silica rings that connect above to a large cation like sodium and below to a layer of metal ions and hydroxyls or halogens, similar to a kaolin fragment.

This layer then bonds to three triangular borate ions. These units link end to end into columns, and each column joins with two others offset by one-third and two-thirds of a unit’s height, forming bundles of three. Color varies with composition: iron-rich specimens are black, bluish-black, or deep brown; magnesium-rich ones are brown to yellow; and lithium-rich types can be blue, green, red, yellow, pink, or rarely colorless. Bicolor and multicolor crystals are common, such as green at one end and pink at the other, or green outside and pink inside, called watermelon tourmaline. Some varieties are dichroic, shifting color depending on the viewing angle.

Localities

Gem and specimen tourmaline is mined chiefly in Brazil and many parts of Africa, including Tanzania, Nigeria, Kenya, Madagascar, Mozambique, Malawi, and Namibia. It is also mined in Asia, notably in Pakistan, Afghanistan, Indonesia, Sri Lanka, and India, where some placer deposits yield gem material. In the United States, the first discoveries were in 1821 in Maine, when two college students found tourmaline on Mount Mica in Paris, Maine. In 1972, the Dunton Quarry on Plumbago Mountain in Newry yielded "The Big Pocket" containing two metric tons of tourmaline, including green, red, and watermelon specimens.

Notable Maine tourmalines include a flawless 256-carat blue gem and a 69-carat gem purchased by Tiffany & Co. California deposits are known for bright pinks and bicolors; during the early 1900s, Maine and California were the world's largest producers of gem tourmalines. The Empress Dowager Cixi of China bought large quantities of pink tourmaline from the Himalaya Mine in San Diego County, California. The first documented California find was in 1890 by Charles Russel Orcutt at what became the Stewart Mine at Pala, California.