Calcium in Our Time

In the earth s crust calcium is ranked fifth in order of abundance among the elements. It is common in all natural waters and essential for aU Hfe. Most calcium is present in combination with sodium, magnesium and aluminum in siHcate minerals. During weathering of siHcates, calcium and sodium ions are most easily released. Unlike sodium, calcium forms compounds with low solubihty with different anions, including carbonate COj , sulfate SO , fluoride F , and phosphate PO . The corresponding minerals are common in sedimentary rocks. Different minerals and their formulas are shown in Table 14.3. 

Marl is the designation for loose Hmestone, earthy or powdery, usually impure calcium carbonate precipitated on the bottoms of ponds largely through the chemical action of aquatic plants. The calcium carbonate in marl may range from 90% to less than 30%. 

Marble is a type of recrystallized limestone. The recrystallization process probably occurred about 2000 million years ago in connection with volcanic activity. The carbonates were heated to a high temperature and simultaneously at such a high pressure that decomposition was avoided. Under these circumstances calcite and dolomite were recrystallized to marble. Marble is a fine-grained rock, although the grains may be very small, almost invisible, as in statuario, the completely white Carrara marble. The white semi-transparent Greek marble from the island of Paros was the material for many famous statues. 

Fluorite CaF is found in many different colors. It occurs in veins, usually as a gangue mineral associated with lead, tin and zinc ores. It is the principal ore of fluorine, and 

Gypsum is a widely distributed mineral and the commonest sulfate mineral, frequently associated with haHte and anhydrite in beds, inter-stratified with limestone, shale and clay. 

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