Tourmaline

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. The Si₆O₁₈ in its formula 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 (as are benitoite, beryl and eudialyte).

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.

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

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.

Other Primary Minerals
Other Hydrothermal Vein Minerals
Other Metamorphic Minerals