Volcanic glass is an isotropic non-crystalline matter (thus, no birefringence is observed). Refractive indices are high for volcanic glasses from basalts and low for glasses found in felsic rocks:
Basic volcanic glass n=1.506-1.612
Felsic volcanic glass n=1.48-1.52
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| Basic volcanic glass with crystallites and crystals of Cpx. Basalt, Washburn Butte, Oregon, USA |
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