Brucite, a mineral consisting of magnesium hydroxide, Mg(OH), and crystallizing in the rhombohedral system. It was first described in 1814 as "native magnesia" from New Jersey by A. Bruce, an American mineralogist, after whom the species was named by F.S. Beudant in 1824; the same name had, however, been earlier applied to the mineral now known as chondrodite. Brucite is usually found as platy masses, sometimes of considerable size, which have a perfect cleavage parallel to the surface of the plates. It is white, sometimes with a tinge of grey, blue or green, varies from transparent to translucent, and on the cleavage surfaces has a pronounced pearly lustre. In general appearance and softness (H = 2½) it is thus not unlike gypsum or talc, but it may be readily distinguished from these by its optical character, being uniaxial with positive birefringence, whilst gypsum is biaxial and talc has negative birefringence. The specific gravity is 2.38-2.40. In the variety known as nemalite the structure is finely fibrous and the lustre silky: this variety contains 5 to 8% of ferrous oxide replacing magnesia, and has consequently a rather higher specific gravity, viz. 2.45. Another variety, manganbrucite, has the magnesia partly replaced by manganous oxide (14%), and thus forms a passage to the isomorphous mineral pyrochroite, Mn(OH).

Brucite is generally associated with other magnesian minerals, such as magnesite and dolomite, and is commonly found in serpentine, or sometimes as small scales in phyllites and crystalline schists; it has also been observed in metamorphosed magnesian limestone, such as the rock known as predazzite from Predazzo in Tirol. The best crystals and foliated masses are from Texas in Pennsylvania, U.S.A., and from Swinaness in Unst, one of the Shetland Isles. Nemalite is from Hoboken, New Jersey, and from Afghanistan. At all these localities the mineral forms veins in serpentine.

(L. J. S.)