Chlorite Mg,Al silicate family of micaceous minerals

Optical Properties:

Biaxial; depending on the composition and chlorite variety, chlorite may be positive or negative; 2V 0-30 degrees; colorless to pale green pleochroism; low birefringence (first order colors, though occasionally anomalous blue); typically occurs as tiny flakes.

The photos above show chlorite filling a vesicle in basalt. Plane polarized light on the left, crossed polarizers on the right.

Occurrence:

Chlorite is a common alteration of ferromagnesian minerals; it is abundant in many low grade metamorphic rocks.