Monday, May 25, 2009

May 2009 Mineral of the Month


From the Cingalese word"tourmali," this was the name given to colored gem 'zircons' found on the island of Sri Lanka (Ceylon). It wasn't until the early 1800s that it was discovered that some of these "zircons" arriving in European gem centers from the far east were actually a previously un-described mineral.

The trigonal crystals formed by this group are often vertically striated. These crystals may be rounded triangular in cross-section. It also forms in massive and compact habits. Seven distinct species make up the Tourmaline group. They are Elbaite (multi hued), Schorl (black), Buergerite and Dravite (brown), Rubellite (pink), Chromedravite (green) and Uvite (black, brown, yellowy green). Crystals are often pink at one end and green at the other and may be of considerable size. Tourmalines form in granites and pegmatites as well as in some metamorphic rocks. They may be found with a wide array of minerals including beryl, zircon, quartz and feldspar.

Tourmaline is a member of the Silicates group. It has a colorless streak, is transparent to opaque and has a vitreous luster. It has a very distinct cleavage and an uneven to conchoidal fracture. Tourmaline has a hardness of 7 to 7-1/2 on theMohs scale. It's complicated chemical compsoition is as follows: Na(Mg.Fe,Li.Mn.Al)3Al6(BO3)3Si6.O18(OH.F)4

This group is insoluble in acids. The darker minerals tend to fuse with more difficulty than the red and green varieties.