Riebeckite thin section

Relief: High positive
Habit/Form: Riebeckite commonly forms slender bladed or prismatic crystals, usually without terminations. Cross sections show the typical diamond shape of amphiboles and the trace of the two cleavages. Grains may be irregular and poikilitic, enclosing associated minerals. The fibrous to asbestiform variety crocidolite is common in metamorphosed iron formations.
Color: Inky blue-black to muddy brown; blue, indigo, violet, yellowish green, yellowish brown, dark blue. Glaucophane tends to show more pale lavender, and riebeckite shows more dark blue-green.
Pleochroism: Intense; strong in blue, yellow-green, brown.
Cleavage: Good in two directions {110} – in two directions are at nearly obtuse angles (124° with 56°), and in one direction for longitudinal section. Sections parallel to (100) which display no cleavage since both cleavages are at too acute an angle to the section to be visible.

Isotropy/Anisotropy: Anisotropic
Interference color: Order I red to order III green; the interference colors are masked by the deep blue color of the mineral.
Extinction angle: The maximum extinction angle in elongate sections is about 0-8°, but the fibrous variety, crocidolite, has parallel extinction.
Twins: Simple and lamellar twins on {100}
Uniaxial/Biaxial: Biaxial (+) (-)
Optic axial angle (2V): 2V measured: 68 – 85°, calculated: 62 – 78°