PYROXENE, in mineralogy, an important group of rock f orming minerals very similar in chemical composition and gen eral characters to the amphiboles (q.v.). The name (from Gr. irvp, fire, and stranger) was originally given by Haily in 1796 to the augite (q.v.) of the lavas of Vesuvius, in the belief that the crystals had been caught up by the lavas that contained them ; but next to the felspars the pyroxenes are the commonest constitu ents of igneous rocks, being especially characteristic of those of basic composition (basalt, gabbro, pyroxenite, etc.). Though crys tallizing in three different systems, the pyroxenes all possess dis tinct prismatic cleavages, with an angle of about 87° between them. Usually the habit of these minerals is that of short prisms, but in the alkaline varieties long slender prisms are often charac teristic, e.g., acmite.
The pyroxenes are conveniently subdivided into three series ac cording to the symmetry of their crystals. ,Chemically they are essentially metasilicates. The principal species are indicated in the table on p. 8o1.
Many of these species are described under their respective headings, to which reference should be made. Others not so treated are briefly mentioned below. Clinoenstatite, the mono clinic magnesium metasilicate, occurs in meteorites. It is the only stable metasilicate of magnesium prepared in the laboratory. It melts incongruently at 1,557° C with separation of forsterite. Enstatite-diopside or magnesium diopside is an isomorphous mix ture of diopside with clinoenstatite, and as such is a characteristic pyroxene of the quartz-dolerite group of igneous rocks. Heden
bergite is a black or dark green pyroxene characteristic of metaso matically metamorphosed lime tones or skarns, and is often asso ciated with andradite garnet. A fluorine-bearing diopside known as mansjoite, occurs at Mansjo Mt. in Sweden. The manganese and zinc-bearing monoclinic pyroxenes include schefferite and jeffersonite, both occurring at Franklin Furnace, New Jersey.
Jadeite is a white to grey pyroxene occurring in dense fibrous masses in association with serpentine, glaucophane schists or eclogites in Turkistan, Tibet, Burma and other places. It pos sesses a distinct ethnographic interest owing to its use by prehis toric man in the manufacture of images and utensils. Pectolite occurs as a secondary mineral in the form of radiating fibrous crystals in druses and veins of basic igneous rocks of the triclinic series of minerals classified as pyroxenes, bustamite, fowlerite and pyroxmangite are closely related to rhodonite. The two former occur at Franklin Furnace, New Jersey. Sobralite occurs with manganese-garnet in an eulysite from Sodermanland in Sweden. According to the most recent analyses, babingtonite contains alkalis and water as essential constituents. The mineral occurs in black crystals on felspar in the granite of Baveno, Italy, and also as a product of contact metamorphism in Japan and elsewhere. The rare zirconium-bearing silicates, hiortda/ilite and the mono clinic rosenbuschite, occur in the nepheline-rich rocks of southern Norway and of the Los archipelago (Guinea). (C. E. T.)