Calcite
. Calcite, an extremely abundant mineral composed of calcium carbonate. It can form crystals in a wide variety of shapes and colors. It can be a primary or secondary component in sedimentary, igneous, or metamorphic rocks. It often provides the cement that binds particles together in sedimentary rocks. Calcite exhibits several physical properties that make it relatively easy to identify. These properties include its tendency to react with a dilute solution of hydrochloric acid and to break into rhombohedrons. Rhombohedrons are six-sided solids that resemble cubes except that the faces meet at 60° instead of 90°. Calcite crystals and calcite-rich rocks are valuable for a variety of uses that range from components in optical instruments to cement. Calcite is the third most common mineral in the earth’s crust (behind feldspar and quartz). Because of its abundance, calcite can be found in many rock types. As a crystal, calcite can take on a variety of forms, also called “habits”. More than