1. Calcite

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, C02 escaping with effervescence. When pure, as in Iceland spar, the mineral is colourless, 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. It is soluble in water holding C02, affording calcium bicarbonate which is found in nearly all natural waters. It is widely diffused among the rocks, and in a state of varying purity forms great masses of limestone.

2. Aragonite

Aragonite (CaC03) is somewhat harder and heavier than 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 less stable as a rule; when heated it is converted into calcite and falls into tiny rhombohedrons of that mineral.

3. Dolomite

Dolomite is a carbonate of lime and magnesia (Ca,Mg) C03; it resembles calcite in appearance, and crystallizes in rhombohedrons which of ten 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.

4. Gypsum

Gypsum, hydrated sulphate of lime, CaS04, 2 H2O. 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, gypsum is transparent and colourless, but is often stained by iron. This mineral occurs largely in granular masses, from which plaster of Paris is made by calcining the gypsum and so driving off the water of crystallization. Alabaster is a gypsum of especially fine grain, mottled in pale colours, or white. Selenite is a transparent variety.

5. Anhydrite

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.

6. Apatite

Apatite is a phosphate and chloride or fluoride of calcium, 3(Ca3P208), 2(Ca, CI, F). Sp. gr. = 2.92-3.25; H = 5. It crystallizes in hexagonal prisms, terminated by hexagonal pyramids, and also occurs in masses. It is sometimes transparent and colourless, but more commonly opaque brown or green. Apatite is soluble in acids, and in water containing carbon dioxide, or ammonia; and gives rise to a valuable plant food.

7. Fluorite

Fluorite, 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, fluorite is either clear and colourless, or blue, green, yellow, or brown.