Olivine (Mg,Fe)2SiO4
Biaxial + or -; very high 2V. Usually colorless in plane light, but Fe-rich varieties have green pleochroism; poor cleavage in one direction; parallel extinction; up to third order interference colors. Commonly has conchoidal fractures and some alteration to serpentine or talc. Distinguished from orthopyroxene by high birefringence, and from clinopyroxene by parallel extinction.
The photos above (crossed polarizers on left, plane light view on right) show the usual appearance of olivine in ultramafic rocks: curved fractures, andhedral grains, high relief, high birefringence.
Olivine is abundant in most ultramafic rocks and is a common constituent of many basalts and gabbros. It is less abundant in diorites and andesites.
The photos below are a dunite on the left (>90% olivine), showing the mineral's high birefringence; on the right is a mixture of olivine and twinned plagioclase, showing the difference in interference colors of the two minerals.
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