Dolomite Composition,
Crystallization & Structure

Composition. Carbonate of calcium and magnesium, CaMg (CO3)₂ = Carbon dioxide 47.8 lime 30.4, magnesia 21.7.varieties occur in which the proportion of CaCO3 to MgCO₃ is not as 1 : 1 Small amounts of ferrous carbonate frequently replace some of the magnesium carbonate. Manganese is also present at times.

Crystallization. Hexagonal- rhombohedral, crystals are usually the unit rhombohedron (cleavage rhombohedron) (Fi. 283). Faces often curved , and sometimes so acutely as to form “saddle-shaped” crystals (Fig. 284) Other forms rare.

Structure. In coarse, granular, cleavage masses to finegrained and compact and in crystal.

Physical Properties. Perfect rhombohedral cleavage (cleavage angle = 106° 15’). H. = 3.5-4 . G. = 2.85. Vitreous luster; pearly in some varieties (pear spar). Color usually some shade of pink, flesh color; may be colorless, white gray, green, brown and black. Transparent to translucent.

Tests. Infusible. After intense ignition a fragment will give an alkaline reaction to moistened test paper. Readily soluble, with effervescence in hot hydrochloric acid; fragment only slowly attacked by cold dilute aced (difference from calcite). Solution oxidized by nitric acid and then made Ammoniacal (may precipitate ferric hydroxide) will with ammonium oxalate give a white precipitate of calcium oxalate; filtrate with sodium phosphate gives granular white precipitate of ammonium magnesium phosphate. Crystallized variety told by its curved rhombohedral crystals and usually by its flesh-pink color.

Occurrence. Dolomite occurs chiefly in widely extended rock masses as dolomite limestone and marble. Occurrence same as for calcite rocks. The two varieties can only be told apart by tests, the simplest being to see if a drop of cold hydrochloric acid placed on the rock will produce effervescence (if so, rock is calcite; if not, dolomite). Often intimately mixed with calcite. Dolomite as a rock mass is thought to be secondary in origin, having been formed from ordinary limestone by action of solutions containing magnesium. Occurs also a vein mineral, chiefly in the lead and zinc veins that traverse limestone. Found in large rock strata in the dolomite region of southern Tyrol; in crystals from the Binnenthal, Switzerland; Traversella in Piedmont; northern England; Guanajuato, Mexico Joplin; Missouri, etc.

Use. At a building and ornamental stone. For the manufacture of certain cements, For the manufacture of magnesia used in the preparation of refractory linings of the converters in the basic steel process.

Ankerite, CaCO₃ (Mg, Fe, Mn) CO₃, is a subspecies intermediate between calite; dolomite and siderite.