Calcareous Minerals
- Calcite, carbonate of lime, CaC03, Sp. gr. = 2.72; H = 3. This mineral crystallizes in the hexagonal system, in a great variety of forms; rhombohedrons and scalenohedrons are common; hexagonal prisms and pyramids less so. Cleavage is very perfect, parallel to the faces of a rhombohedron, and the mineral breaks up into rhombohedrons when struck a sharp blow. Calcite is rapidly attacked, even by cold and weak acids, CO2 escaping with effervescence. When pure, as in Iceland spar, the mineral is colorless, very transparent and lustrous, and displays the phenomenon of double refraction strongly; but more commonly it is cloudy or white, or stained red or yellow by iron. The decomposition of silicated minerals containing lime gives rise to calcite, and as this is soluble in water holding CO2, nearly all natural waters have more or less of it in solution. It is widely diffused among the rocks, and in a state of varying purity forms great masses of limestone.
- Aragonite (CaC03) is a somewhat harder and heavier form of calcite, with a specific gravity of 2.93 and a hardness of 3.5-4, and crystallizes in compound prismatic forms which belong to the orthorhombic system. It has not the marked cleavage of calcite and is very unstable; when heated it is converted into calcite and falls into tiny rhombohedrons of that mineral.
- Dolomite is a carbonate of lime and magnesia, (Ca, Mg)CO3, but with variable proportions of the two bases; it resembles calcite in appearance, and crystallizes in rhombohedrons which often have curved faces. Sp. gr. = 2.8-2.9; H=3.5-4. Dolomite may be readily distinguished from calcite by the fact that cold acids affect it but little.
- Selenite, hydrated sulphate of lime, CaS04, 2 H20. Sp. gr. = 2.31-2.33; H = 1.5-2. It crystallizes in right rhomboidal prisms, belonging to the monoclinic system, and cleaves into thin non-elastic leaves. When pure, selenite is transparent and colorless, but is often stained by iron. This mineral occurs largely in granular masses, called gypsum, from which plaster of Paris is made by calcining the gypsum and so driving off the water of crystallization. Gypsum is slightly soluble and is present in most natural waters. Alabaster is a gypsum of especially fine grain, mottled in pale colours, or white.
- Anhydrite, CaS04, is sulphate of lime without water; it is harder and heavier than gypsum (Sp. gr. = 2.9-2.98; H = 3-3.5), and crystallizes in a different system, the orthorhombic. The crystals have three sets of cleavage planes, which intersect each other at right angles.
- Apatite is a phosphate of lime, with chloride and fluoride of calcium, which vary in relative amounts, 3 (Ca3P208), 2 (Ca, Cl, F). Sp. gr. = 2.92-3.25; H = 5. It crystallizes in hexagonal prisms, terminated by hexagonal pyramids, and also occurs in masses The original and unchanged mineral is transparent and colorless, changing on alteration to opaque brown or green. Apatite is soluable in acids, and in water containing carbon dioxide, or ammonia i it is thus dissolved out of the rocks and widely diffused in the soils, where it forms a valuable plant food.
- Fluor-spar, fluoride of calcium, CaF2. Sp. gr. = 3.01-3.25; H = 4. Crystallizes in the isometric system, usually in cubes, and has a perfect octahedral cleavage. When pure, fluorspar is clear and colorless, but it is more commonly stained blue, green, yellow, or brown.