Isotropism, Anisotropism. Most minerals other than amorphous and those crystallizing in the isometric system are anisotropic.

Interference Color. If the thickness of the section is known, interference colors can be used to estimate birefringence, a useful diagnostic characteristic. In strongle colored minerals, interference colors may be masked or altered by th ecolor of the mineral. If several grains of the same mineral are present, the highest interference color observed is most diagnostic of the mineral.

Extinction. For elongated mineral grains, the angle of extinction should be measured carefully on several individuals. If it corresponds to 0° or 90°, the mineral has parallel extinction, else it is inclined. If inclined, the highest measured value is noted, as mineral orientation in thin section may influence extinction angle.

Elongation. The orientation of the slowest and fastest rays can be determined by inserting a compensator accessory plate. This property can only be determined on elongate grains, an dis not always an absolute criterion.

Twinning. This may be deduced from the relative pattern of extinction of the mineral grain. Twinning pattern is an important diagnostic feature of feldspars and other minerals. In some minerals (orthoclase) the twinning may not be observed.

This page (http://www.ces.ncsu.edu/plymouth/programs/cross1.html) created by
Vera MacConnell, Research Technician, I on November 17, 1997.
Last Updated on November 17, 1997.