Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


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 300 different forms of calcite exist. Some especially common shapes are “dogtooth spar” and rhombohedrons. Dogtooth spar crystals are elongated six-sided pyramids, except without a flat bottom. Rhombohedrons resemble cubes except that their faces intersect at 60° instead of 90°. Most large crystals of calcite form in veins by precipitation from hot or cold groundwater as it moves through cracks in rock deep underground. Most large masses of calcite-rich rock form biochemically, in the case of limestone, or as a precipitate, in the case of travertine.