Chromite Composition, Crystallization & Structure

Composition. FeCr2O3 = Chromium sesquioxide 68.0, iron protoxide 32.0. The iron may be replaced by magnesium and the chromium by aluminum and ferric iron.

Crystallization. Isometric. Habit octahedral. Crystals small and rare.

Structure. Commonly massive, granular to compact.

Physical Properties. H. = 5.5  G. = 4.6. Metallic to submetallic luster. Color iron-black to brownish black. Streak dark brown.

Test. infusible. When finely powdered and fused on charcoal with sodium carbonate gives a magnetic residue. Imparts a green color to the borax and salt of phosphorus bead (chromium).

Occurrence. A common constituent of peridotite rocks and the serpentines derived from them. One of the first minerals to separate from a cooling rock magma, and its large ore deposits are thought to have been derived by such magmatic differentiation.  Associated with chrysolite, serpentine, corundum, etc.

Found only sparingly in the United States. Pennsylvania, Maryland, North Carolina and Wyoming have produce it in the past. California is the only important producing state a t p resent (1928). The important countries for its production are New Caledonia, Southern Rhodesia, Greece and Canada.

Uses. Chromium is used with various other metals to give hardness to steel. Chromite bricks are used to a considerable extent as linings for metallurgical furnaces, on account of their neutral and refractory character. The bricks are usually made of crude chromite and coal tar but sometimes of lime or with other materials. Chromium is a constituent of certain green, yellow, orange and red pigments and of similarly colored dyes.