Biaxial calcite: Occurrence, optics, and associated minor strain phenomena

Abstract

Biaxiality in calcite (2 V≤10°, less commonly ∼15°) is widespread in metamorphic rocks of all grades and in carbonatites. It is a minor effect of late strain, in many cases not directly related to visible {01¯12} twinning, and partially independent of lattice bending. For values of 2V up to about 15° the optic axial plane coincides, within limits of error of measurement, with {10¯10}. For those rare crystals with 2V in the range 25–50°, there is a real departure—never greater than 10°—from this orientation; and biaxiality is certainly in some, probably in all such cases, confined to domains of overlap of single thin twin lamellae. Microscopic procedure is described for determining the orientation of the optical indicatrix relative to crystallographic coordinates.