Definition
tourmaline: A group of minerals composed of complex cyclosilicates containing boron with the general formula NaR
3 2+Al
6B
3Si
6O
27(H,F)4, where R = Fe
2+, Mg, or (Al + Li). The crystals are trigonal, elongated, and variably coloured, the two ends of the crystals often having different colours. Tourmaline is used as a gemstone and because of its double refraction and piezoelectric properties is also used in polarizers and some pressure gauges.
Agricultural Uses
Tourmaline, which is a borosilicate, is the main source of
boron found in soil. It is a mixture of silicates and borates
from where boron is extracted. It is available as a
complex cyclosilicate, Al
6Y
3Na[Si
6O
18](BO
3)
3(OHF)
4,
with Y = Mg, Fe, Mn or Li. The rhombohedral system
has the shape of an elongated rod or needle type prism.
Tourmaline is used as a gemstone, and because of its
double refraction and piezo-electric properties, it is used
in polarizers and in some pressure gauges. The best
known variety of tourmaline is black tourmaline which
is rich in iron and is a common mineral in igneous and
metamorphic rocks.
Magnesium-rich tourmaline is called dravite.
Lithium-rich varieties are called elbaite. Ferruginous
varieties are called schorlite which is insoluble in water
and resistant to weathering and hence releases boron
slowly.
