guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Double refraction
4.2.3 Double refraction
In non-cubic, optically anisotropic crystals, light propagates in all directions as two sets of
orthogonally vibrating plane-polarized waves, except for directions parallel to an optic axis.
The two waves have different velocities (and correspondingly different refractive indices n z '
and n x ').
The specific relation between crystal symmetry and indicatrix in each mineral species defines
the orientation of the polarization plane and the refractive index of the two waves (Ch. 4.1.2;
Figs. 4-6,7). For every crystal section seen in thin section, these parameters can be derived
from the ellipse created by the intersection of the indicatrix and the thin section plane (Fig. 4-
21).
Raith, Raase & Reinhardt – February 2012
Figure 4-21. Relationship between crystal section, crystal orientation and indicatrix
orientation using melilite as an example.
80