Tourmaline (Schorl)

Schorl (also spelled schörl) is the common black form of tourmaline. Not that tourmaline is a common mineral, but if you see it at all it's usually black. Schorl tourmaline has the formula NaFe²⁺₃Al₆(BO₃)Si₆O₁₈(OH)₄. The Si₆O₁₈ signifies that silicon atoms, surrounded by four oxygen atoms in silica tetrahedrons, occur linked into rings of six, each sharing oxygens with their two neighbors. Thus tourmaline is a cyclosilicate or ring silicate mineral (benitoite is another).

Tourmaline is easy to recognize. This Chinese tourmaline specimen, shown about twice life size, displays the typical crystal shape of a nine-sided prism. It also displays the typical striations along the long axis and the glassy luster. Its hardness is 7 to 7.5, its streak is white, and it has poor cleavage. Tourmaline might be mistaken for hornblende, another black glassy mineral that occurs in prisms, but hornblende forms flattened crystals (being an amphibole), is softer (5 to 6), and has a strong cleavage.

Clear and colored forms of tourmaline can be attractive gemstones and fancy specimens; their chemical formulas replace Na with Ca and Fe with Mg, Al, or Li; they may also contain fluorine in place of the OH group. But all forms of tourmaline display piezoelectricity, which means that they change their electrical properties in response to pressure. This makes tourmaline useful in certain electrical apparatus. Tourmaline also gains an electric charge upon heating and cooling—you can watch the crystal pick up bits of paper as this happens.

Tourmaline is found in coarse-grained granite bodies, especially their late-forming pegmatite cores, and in some metamorphic rocks. It is the principal boron mineral in these rocks, whereas in sedimentary rocks boron occurs as borates like ulexite.

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