Doubly Terminated Schorl Tourmaline Specimens
The tourmaline mineral group is a chemically complicated group of borosilicate minerals having the general composition WX3Y6[Si6O18](BO3)3(O,OH,F)4. The tourmaline group consists of over 30 closely related mineral species - including dravite, elbaite, schorl and uvite, the four most common and well known tourmaline species. As a mineral group, the tourmaline species share a 6-ring cyclosilicate structure in common, but differ from one another in varying isomorphous replacements of calcium, sodium, potassium, lithium, magnesium, iron, manganese, aluminum, chromium, and vanadium in the end members of solid solution series that variously grade into one another.
Tourmalines are characteristic minerals in granitic pegmatites and are common accessory minerals in granite, granodiorite, and related felsic igneous rocks. Tourmalines occur in rocks that have been hydrothermically altered adjacent to pegmatite and related felsic intrusives, and are common accessory minerals in gneiss, phyllite, quartzite and schist. The complex chemistry of tourmaline group minerals reflects diverse compositions of both host rock and hydrothermal fluids, as well as differences in temperature and pressure of formation.
Tourmalines are systematically classified in the trigonal division of the hexagonal crystal family. Prismatic tourmaline crystals commonly form as stubby to elongate vertically striated prisms with a somewhat rounded, triangular cross section. Terminations are typically blunt and consist of flat plane tops or trigonal pyramids. Tabular, radiated, bladed and massive habits also occur.
Due to their non-centrosymmetric crystal structure, tourmaline crystals can exhibit both pyroelectricity (become electrically charged when heated or cooled) and piezoelectricity (become electrically charged when mechanically stressed). The non-centrosymmetric crystal structure of tourmaline can also result in hemimorphic (not symmetrically equivalent) terminations on opposite ends of doubly terminated crystals.
Tourmalines occur in virtually every color including colorless, black, bi-colored and tri-colored crystals. Schorl, a sodium-iron rich end member in the tourmaline group with the composition NaFe3Al6[Si6O18](BO3)3(O,OH,F)4, is the most ubiquitous tourmaline species and is characteristically black in color with a vitreous luster. Schorl enjoyed some popularity as a gemstone in Victorian era mourning jewelry and is currently regaining popularity as a gem. The play of reflected light on the surface of a well faceted and polished schorl can make for a unique and surprisingly captivating gemstone. Schorl crystals are amongst the most attractive black mineral specimens and are popular with mineral collectors.