A mineral consisting largely of calcium carbonate (CaCO3 ). Next to quartz, it is the most abundant of the Earth's minerals. Crystallizing in the hexagonal system, calcite is noted for its wide variety of crystalline forms.
Calcite crystals, Pough Quarry, Ohio.
Calcite is colourless or white when pure but may be of almost any colour - reddish, pink, yellow, greenish, blueish, lavender, black, or brown, owing to the presence of diverse impurities. It may be transparent, translucent, or opaque. Its luster ranges from vitreous to dull; many crystals, especially the colourless ones, are vitreous, whereas granular masses, especially those that are fine-grained, tend to be dull. Calcite is number 3 on the Mohs hardness scale; it can be scratched readily by a knife blade or geologic pick. It has a specific gravity of 2.71. Three perfect cleavages give calcite its six-sided polyhedrons with diamond shaped faces; the angles defining the faces are 78 and 102 degrees.
Calcite showing double refraction
When light passes through some minerals, it is split into two rays that travel at different speeds and in different directions. This phenomenon is known as birefringence. Calcite consequently exhibits double refraction that can be observed with the naked eye. Iceland spar, first produced in the 17th century from the east coast of Iceland, has been used for nearly two centuries for optical instruments. William Nicol in 1828 found that by cutting the crystals in the appropriate direction one could make an optical device which eliminated one of the rays and permitted the other to emerge as plane polarised light. Polaroid sunglasses use this same property to cut down glare on a sunny day. This technology was used in microscopes which are utilized in the study of rocks and minerals.
From an investigation of the cleavage and crystal forms of calcite, RenŽ J. Hay developed (1781-1801) a theory of crystal structure that played an important part in the evolution of modern structural crystallography. When a friend accidentally shattered a prized calcite crystal by dropping it on the floor, HaŸy, was shocked to find it broke into rhombic shapes. He continued to break other pieces of calcite and was able to determine the idea of crystal structure which came from the calcite breaking in a similar manner each time.
Calcite crystal, Dalnegorsk, Siberia (from the Paul and Janet Clifford Collection).
Uses for Calcite:
Limestone is a rock made of calcite. Most limestone is grey, but all colours of limestone from white to black have been found. Scientists test natural rock to see if it is limestone by pouring cold diluted hydrochloric or sulphuric acid (10% solution or vinegar) on it. Limestone gives off bubbles of carbon dioxide.
Most fresh water and sea water contain dissolved calcium carbonate. All limestones are formed when the calcium carbonate crystallizes out of solution. Or from the skeltons of small sea urchins and coral All the different kinds can be divided into two groups.
The first group includes limestones that formed almost completely without the aid of organisms. This type of limestone is forced out of solution when the water evaporates. Such evaporation takes place in the hot lagoons of many coral reefs, and in most shallow tropical seas. The high temperature causes the water on the surface to evaporate. A white 'lime' mud is deposited on the bottom of the sea. This white mud slowly hardens into a light coloured limestone that remains soft. When spring water evaporates on land, calcium carbonate forms a crust over moss, dead leaves, and the ground. It builds up a mound or terrace called tufa. Evaporation of water in limestone caverns forms another variety of limestone, called stalactites and stalagmites.
Limestone containing stems of crinoid fossils
Almost everyone who has visited a cave or who has seen pictures of them seems to be more impressed by the curious formations in them than by the cave itself. From the roof of the cavern may depend long slender 'icicles' of stone; and upward from the floor spring similar, but generally more squatty, grotesque deposits. The 'stonecicles' that hang from the ceiling are called stalactites, and those that rise from the floor are stalagmites. In some cases they join; others are sheet-like instead of cylindrical; but one and all are due to the redeposition of calcite by ground water.
If rain water, circulating downward through the rocks above a cavern, dissolves some calcite along the journey, it may drip from the roof of the cave. As it reaches the air it evaporates; and when it does, calcite is deposited. By a slow process stalactites are formed and elongated. Meanwhile, the water that drips to the floor may evaporate there, and thus build up a stalagmite. As usual, this simple picture is complicated in nature, and we find that relative concentrations of carbon dioxide, pressures, and especially temperature change enter into the interplay of factors that decide whether the ground water is to be in a dissolving or depositing mood. Again we must forego technicalities, but we can explain that if the environment entered by the water is one of relatively high carbon dioxide concentration, solution will have the tendency, whereas a lower concentration of carbon dioxide will cause deposition. There really is a nice balance here, and it affords one more example of the approaches toward equilibrium that Nature seems so anxious to achieve.
The second group of limestones forms by the work of organisms. Many aquatic organisms draw calcium carbonate out of the water and use it to make their shells and bones. The oysters, clams, snails, coral and sea urchins do this. When animals die; the shell and bones are broken down by the waves into shells and coral sand and mud. Many beaches on Pacific islands are made of such coral mud and sand. Most of the limestone layers in all parts of the earth were once shell or coral sand and mud. A limestone called coquina, formed of shells and coral, occurs in Florida.
Uses of Limestone:
Marble is a metamorphic rock formed by the alteration of limestone by heat and pressure. The calcite in the limestone changes and fossils and layering in the original limestone disappear as interlocking grains grow. If the limestone is pure, a white marble is formed. Limestones may include layers of clay or sand which may form the attractive flow banding and colours found in decorative marble.
Marble Quarry, Proctor, Vermont, U.S.A.
Uses of Marble: