Prehnite H2Ca2Al2(SiO4)3

Optical Properties:

Biaxial +; variable 2V; moderate birefringence, up to high second order colors;colorless in plane polarized light; 1 good cleavage; high relief; parallel extinction; tabular crystals.

The photos above illustrate typical prehnite occurrences, in veins in low grade metamorphic rocks. On the left, in plane polarized light, is a thin vein with pumpellyite and clear, elongate crystals of prehnite. On the right, under crossed polarizers, prehnite birefringence is displayed. Both photographs have a field of view of 1.5 mm.

Occurrence:

Occurs in the matrix and in veins in low temperature metamorphic rocks, and as a vesicle filling in some lavas.