Calcite

I

Introduction

Calcite, an extremely abundant mineral composed of calcium carbonate (CaCO3). 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.

II

Composition

Calcite contains the carbonate ion (CO3-2), making it part of the carbonate family of minerals, which also includes dolomite, azurite, and aragonite. Calcite differs from dolomite and azurite in chemistry and from aragonite in internal structure. In addition to the carbonate ion, calcite contains calcium whereas dolomite contains an equal mixture of calcium and magnesium and azurite contains copper.

Calcite and aragonite have identical chemical compositions, but the molecules are stacked differently in the crystals. When two minerals have identical chemical compositions but different crystal structures, they are polymorphs of each other. Calcite is generally more stable than aragonite. Over several years, most aragonite will change to calcite. Thus, newly created calcium carbonate deposits may contain some or even a lot of aragonite, but ancient deposits contain little or no aragonite.

Calcite is only slightly soluble in pure cold water. However, rainwater or other water in contact with air will absorb carbon dioxide from the air. Carbon dioxide reacts with water to form carbonic acid. Calcite reacts with carbonic acid to form calcium bicarbonate. Calcium bicarbonate is quite soluble in water. Thus, the solubility of calcite is sensitive to the acidity of the water. The solubility of calcite is also sensitive to the temperature of the water, being quite soluble in cold water but not in hot water. Calcite frequently dissolves in one location and precipitates in another location when the water becomes less acidic or cools.

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III

Occurrence

Calcite is the third most common mineral in Earth’s crust (behind feldspar and quartz). Because of its abundance, calcite can be found in many rock types.

A

Calcite in Sedimentary Rocks

The sedimentary rock limestone makes up approximately 10 percent of all sedimentary rock. Limestone consists almost entirely of calcite. It forms because many marine organisms make shells and other hard body parts out of calcium carbonate. When the organisms die, their shells or hard body parts settle to the ocean floor. Through time, they can accumulate into great thicknesses of calcite mud. This mud, if turned into rock, becomes limestone. Chalk, a form of limestone, consists of the calcium carbonate remains of innumerable microscopic floating marine organisms, such as foraminifera and coccolithophores.

Travertine, sometimes called “tufa,” consists of calcite that is precipitated from water in hot or cold springs (see Mineral Deposit). An especially dense type of travertine, called Mexican onyx, is frequently used as decorative stone. Mexican onyx was mistakenly called alabaster in biblical times and is still occasionally called it today.

Cave deposits also typically consist of calcite. They form because groundwater becomes saturated in calcium bicarbonate as it moves through cracks in limestone above the caves. When the groundwater drips from a crack into a cave passageway, some of the water may evaporate into the air in the cave. Even more important, some of the dissolved carbon dioxide may escape to the air in the cave and thereby reduce the acidity of the water. In either case, the water becomes less capable of holding the dissolved material and calcite (or aragonite) precipitates out. Through time, icicle-shaped deposits form on the ceiling of the passageway, called stalactites, and on its floor, called stalagmites (see Stalactite and Stalagmite).

Calcite also provides cement for many clastic sedimentary rocks. Clastic sedimentary rocks consist of innumerable fragments of pre-existing rock, or clasts, held together by cement. Sandstone, for example, is a clastic sedimentary rock consisting of sand particles cemented together. Calcite forms cement when it precipitates from groundwater into pore spaces between clasts. In addition to calcite, the other important cement is silica (see Quartz).

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