Calcite thin section

Relief: Low to high; twinkling; variable relief.
Habit/Form: Crystals of calcite have many habits but usually consist of combination of scalenohedrons and rhombohedrons. However, in most rocks, calcite forms anhedral grains or aggregates of grains. Fossil shells and thin veins may be fibrous or columnar.
Color: Colorless (often cloudy); can be white, grey, pink
Pleochroism: –
Cleavage: Rhombohedral cleavage; at least one cleavage; usually shows at two intersecting lines at oblique angles (75° if section is cut normal to the cleavage traces). In fine aggregates the cleavage may not show. There is sometimes parting parallel to {0112} which is due to twin-gliding.

Isotropy/Anisotropy: Anisotropic
Interference color: High-order; the maximum interference color is pearl gray or white of the higher orders. Thin edges of the slide usually show bright colors of the fourth and fifth orders and tints of higher orders. Thin films and twin lamellae of calcite usually show bright interference colors.
Extinction angle: Symmetrical or inclined to the rhombohedral cleavages. When a section is in one of the extinction position, fine birefringent calcite dust formed by griding is prominent.
Twins: Polysynthetic twinning with {01-12} as twin-plane is very common, especially in the calcite of metamorphic limestone. The twin lamellae are mostly parallel to the long diagonal, but they also intersect at oblique angles depending upon how the section is cut. The twin lamellae are usually so thin that they show first-order interference colors.
Uniaxial/Biaxial: Uniaxial (-)
Optic axial angle (2V): –