Occurrence and Compositon
Calcite and dolomite are common as primary minerals in a wide variety of sedimentary and metamorphic rocks. The may also form as secondary minerals. Calcite is CaCO3; dolomite is CaMg(CO3)2.

These two carbonates are generally colorless, have extreme birefringence, and show one or two sets of polysynthetic twins that may show in both PP and XP views. They are typically told apart by using a chemical stain (alizarin red) that turns calcite pink.

Important properties
 ·Twinning common - depending on grain orientation, one or two sets of parallel twins may show. The carbonates are the only common minerals that typically show twinning in both PP and XP views.
 ·Appearance and habit - When euhedral, calcite and dolomite may appear hexagonal. More typically they are anhedral in aggregates. They show high relief that may vary with stage rotation.
 ·Color - colorless but may be cloudy or show vague pastel hues or twinkling effects in PP views.
 ·Cleavage - typically shows rhombohedral cleavage
 ·Interference colors - very high order, often appearing as "pearl' white
 ·Twinning common - Depending on grain orientation, one or two sets of parallel twins may show.

Similar minerals:
 ·Distinguishing calcite, dolomite and other rhombohedral carbonates from each other can be very difficult without obtaining chemical analysis or using chemical stains.
 ·Aragonite, a polymorph of calcite, is orthorhombic and does not show rhombohedral cleavage.

Calcite (or Dolomite) in a Marble

The views above show calcite (or dolomite) in a marble from the Adirondack Mountains, New York. The field of view is about 2 mm across. Minor graphite (opaque) is also present as long skinny flakes in the carbonate.

Most of this view is one large grain showing two sets of twins. Calcite is invariably twinned but the twins do not show at all orientations. The largest grain is oriented so many of the twin lamellae are at extinction (black in XP). Very high order interference colors -- high order pastels -- show along the (not extinct) twin lamellae. The large grain to the upper left is oriented such that twins do not show. This grain has interference colors that are of such high order that they appear white.

Epidote, Dolomite and minor Pholgopite in a Marble

This is a marble from Essex County, New Jersey. The twinned dolomite is clear (PP) and shows high order, almost white, interference colors (XP). Dolomite, like calcite, is one of the few minerals that shows twins in both PP and XP. Epidote, also present, has greater relief than the surrounding dolomite and has a very light pistachio green color (PP). The epidote shows a typical (for epidote) interference color pattern - blotchy high order colors of many hues. Several flakes of brown-green phlogopite (PP) can also be seen.

The field of view is about 2.5 mm across.