Brucite Composition,
Crystallization & Structure
Composition. Magnesium hydroxide, Mg (HO)2 = Magnesia 69.0, water 31.0 Iron and manganese sometimes present.
Crystallization. Hexagonal-rhombohedral. Crystals usually tabular with prominent basal planes, showing at times small rhombohedral truncations.
Structure. Commonly foliated, massive.
Physical Properties. Perfect basal cleavage. Folia flexible but not elastic. Sectile. H. = 2.5 G. = 2.39. Luster on base pearly, elsewhere vitreous to waxy. Color white, gray, light green. Transparent to translucent.
Test. infusible. B.B. glows. Gives water in C.T. Easily soluble in hydrochloric acid, and after solution has been made ammoniacal an addition of sodium phosphate gives a white granular precipitate of ammonium magnesium phosphate (test for magnesium). Recognized by its foliated structure, light color and pearly luster on cleavage face. Distinguished from talc by its greater hardness and lack of greasy feel.
Occurrence. Found associated with serpentine, dolomite, magnesite, chromite, etc., as a decomposition product of magnesium silicates. Notable localities for its occurrence are at Unst, one of the Shetland Islands; Aosta, Italy; at Tilly Foster Iron Mine, Brewster, New York; at Wood’s Mine, Texas, Pennsylvania.
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