Turning to the “Other” Tourmaline Crystals

COMPLEX, PLENTIFUL, AND AFFORDABLE

- BOB JONES

Of all the mineral groups collectors enjoy, the tourmaline group includes the nicest variety of colorful gem crystal specimens. It also happens to be among the most chemically complex gem species. As an example, uvite, which is not often viewed as colorful, is made up of ten different elements that repeat, which means, a single molecule of uvite is comprised of almost 60 atoms.

Many collectors are particularly attracted to elbaite, the most colorful and popular of the tourmalines. The chemistry of elbaite is also quite complex. One scientist even referred to it as a “garbage can” mineral because it seems to take in any stray elements available within the solution it develops. Tourmaline’s ability to accept trace elements accounts for it acting as chromophores, giving elbaite its variety of colors. Other members of the tourmaline group proceed similarly and are found mainly in pegmatite deposits.

Pegmatite deposits form in the final stages of a deep-seated granitic mass called a batholith, where different elements tend to accumulate. As a batholithcools over millions and millions of years, molecules of like minerals develop, while basic quartz, feldspar, and mica form a matrix. As this happens, the more volatile elements that crystallize at much lower temperatures, including fluorine, boron, beryllium, lithium, uranium, and others, remain longer in solution or as part of the vapor and hydrothermal components under extreme pressure. Such residual fluids can thrust into rock openings forming veins, sheets, or dikes and cool slowly allowing larger than standard crystals of feldspar and quartz to form in an interlocking system called graphic granite.