Tourmaline
This tourmaline crystal is from Lennox Creek, Washington State. A single linear polarizing filter is oriented
at right angles to the length of the crystal. This is parallel to the omega plane and show the highest absorption.
Transmitted Light with a Single Linear Polarizing Filter Oriented Perpendicular to the Long Axis
Definition/Function:
Tourmaline has the general formula of (Na,Ca)(Mg,Fe,Mn,Li,Al)3(Al,Mg,Fe)6[Si6O18](BO3)3(O,OH)3(OH,F).
It is a common mineral in some granites or metamorphic deposits.
Significance in the Environment:
Characteristic Features:
Tourmaline belongs to the trigonal crystal system and is optically negative. Its refractive indices are 1.612-1.650 for epsilon and
1.633-1.671 for Omega. Its birefringence ranges from 0.017 to 0.035. The density of tourmaline ranges from about 2.9 to 3.22 grams/cc.
Small grains may be colorless to black. Their pleochroism is strong if they are strongly colored. The absorption is always strongest along
omega, the highest refractive index. The long axis of the grains tends to be the epsilon direction.
Associated Particles:
References:
Deer, W. A., R. A. Howie, and J. Zussman, AN INTRODCUTION TO THE ROCK-FORMING MINERALS, ISBN 0-582-30094-0, pp. 130-7, 1992.
