Magnesite Composition, Crystallization & Structure

Composition. Magnesium carbonate, MgCO₃ = Carbon dioxide 47.6. Iron carbonate also often present.

Crystallization. Hexagonal–rhombohedral. In rhombohedral crystals.

Structure. Compact earthy forms common, also less frequently in cleavable granular masses, coarse to fine. Crystals rare.

Physical Properties. Perfect rhombohedral cleavage, sometimes distinct. H. = 3.55-4.5. G = 3-3.1. Vitreous luster. Color white, gray, yellow, brown. Transparent to opaque.

Tests. Infusible. After intense ignition gives a faint alkaline reaction on moistened test paper. Scarcely acted upon by cold but dissolves with effervescence in hot hydrochloric acid. Solution after the precipitation of any iron and calcium, gives in the presence of an excess of ammonia, with sodium, with sodium phosphate, a white granular precipitate of ammonium magnesium phosphate.

Occurrence. Commonly derived from the alteration of rocks rich in magnesium, through the action of waters containing carbonic acid. Such magnesites are commonly colloidal in character and often contain opal silica. Crystallized Magnesite is formed in various ways. Large bodies of what is considered to be sedimentary Magnesite have been found in western United States. Associated with serpentine, dolomite, brucite, etc. Magnesite occurs in the Coast Range of California, and in sedimentary beds in Clark Co., Nevada and in the Mohave Desert, California. Most of the Magnesite used in the United States is imported, coming chiefly from Stryia in Austria and from Greece.

Use. Magnesite is chiefly used in the preparation of Magnesite bricks for refractory linings in metallurgical furnaces. Also used in the preparation of magnesium salts (Epsom salts, magnesia, etc.)