Tourmaline ( #5)
|Asia||Paprok, Nuristan, Afghanistan||4" x 3" x 2"|
The chemical composition of tourmaline is very complex. The chemical formula may be written: Na(AI, Fe2+, Fe3+, Li, Mg2+, Mn2+)3(AI, Cr3+, Fe3+, V3+ )6(BO3)3Si6O18(OH)4, with the possibility of even more substitutions that are less common. Elements written with in parentheses may substitute into the same atomic sites in the crystal structure. There are different varieties of tourmaline that are defined by a relative abundance of certain of these elements. For example, this specimen is the variety elbaite, and its general formula is Na(Al,Li)3Al6(BO3)3Si6O18(OH)4.
Tourmaline occurs in many kinds of rocks, but such fine specimens as this and specimen #17 grow only under conditions that are not generally realized. This specimen came from the Paprok Mine in Nuristan Province of Afghanistan. The mine is in rocks called pegmatites, which occur in igneous rocks and are characterized by unusually large crystal sizes. They likely originate in the late stages of crystallization of a magma (a molten mass) when the concentration of volatile elements is high and the rate of nucleation of new crystals is low. This facilitates the transport of chemical elements to the growth surfaces of a relatively few crystals, which can then achieve large size.
While pegmatites are by no means rare, most of them do not yield spectacularlyshowy crystals like this one.