Bronzite

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Bronzite is a member of the pyroxene group of minerals, belonging with enstatite and hypersthene to the orthorhombic series of the group. Rather than a distinct species, it is really a ferriferous variety of enstatite, which owing to partial alteration has acquired a bronze-like sub-metallic luster on the cleavage surfaces.

Enstatite is magnesium metasilicate, MgSiO₃, with the magnesia partly replaced by small amounts (up to about 5%) of ferrous oxide; in the bronzite variety, (Mg,Fe)SiOa, the ferrous oxide ranges from about 5 to 14%, and with still more iron there is a passage to hypersthene. The ferriferous varieties are liable to a particular kind of alteration, known as schillerization, which results in the separation of the iron as very fine films of oxide and hydroxides along the cleavage cracks of the mineral. The cleavage surfaces therefore exhibit a metallic sheen or schiller, which is even more pronounced in hypersthene than in bronzite. The color of bronzite is green or brown; its specific gravity is about 3.2 - 3.3, varying with the amount of iron present. Like enstatite, bronzite is a constituent of many basic igneous rocks, such as, norites, gabbros, and especially peridotites, and of the serpentines which have been derived from them. It also occurs in some crystalline schists.

Bronzite is sometimes cut and polished, usually in convex forms, for small ornamental objects, but its use for this purpose is less extensive than that of hypersthene. It often has a more-or-less distinct fibrous structure, and when this is pronounced the sheen has a certain resemblance to that of cats-eye. Masses sufficiently large for cutting are found in the norite of the Kupferbergin the Fichtelgebirge, and in the serpentine of Kraubat near Leoben in Styria. In this connection mention may be made of an altered form of enstatite or bronzite known as bastite or schiller spar. Here, in addition to schillerization, the original enstatite has been altered by hydration and the product has the approximate composition of serpentine. In color bastite is brown or green with the same metallic sheen as bronaite. The typical locality is Baste in the Radauthal, Harz, where patches of pale greyish-green bastite are embedded in a darker-colored serpentine. This rock when cut and polished makes an effective decorative stone, although little used for that purpose.