TOURMALINE
Tourmaline is a classic "garbage-can
mineral" - it has a little bit of just about everything. Tourmaline
can be given the formula (Na,Ca)(Li,Mg,Al)(Fe,Mn,Al)6(BO3)3(Si6O18)(OH,F)4
- sodium calcium lithium magnesium iron manganese hydroxy-fluoro-boro-aluminosilicate.
Tourmaline has a nonmetallic luster, varies in color but is often blackish, has
a white streak, is quite hard (H = 7 to 7.5), frequently has elongated crystals
with rounded triangular cross-sections and striated faces, no cleavage, and conchoidal
fracture.
Tourmaline is a group of minerals, the most common of
which is the blackish-colored, Fe-rich schorl. The dark brown,
Mg-rich dravite is another moderately common variety of
tourmaline. Other varieties include achroite, elbaite (Li & Na-rich),
indicolite, liddicoatite (Li & Ca-rich), rubellite, verdelite, etc.
The latter tourmalines are often richly colored (greenish, yellowish, reddish,
pinkish, bluish, multicolored).
Tourmaline is moderately common in pegmatites and some
metamorphic rocks. It can even be a rock-forming mineral in tourmalinite.
Schorl tourmaline (large one at lower left is 13 mm across; smaller
sticks are ~10 to ~14 mm long each)
Schorl tourmaline
Dravite tourmaline (above & below; above: 7.3 cm across). These come from the famous Proterozoic-aged, metamorphosed Yinnietharra Granite Pegmatite of Australia at Yinnietharra Station Homestead, Gascoyne River area, southern Pilbara region, Western Australia.
Elbaite tourmaline
Elbaite tourmaline (red) & albite feldspar (white, NaAlSi3O8)
from the Jonas Mine, Itatiaia District, Docey Valley, Conselheiro Pena, Minas
Gerais State, Brazil (CSM # 53677, Colorado School of Mines Geology Museum,
Golden, Colorado, USA).
Rubellite tourmaline - radiating cluster of dark pinkish rubellite in
lepidolite matrix, from the Cretaceous-aged Pala Pegmatite of southern
California, USA.