PYROXENE  , an important

group of rock-forming minerals, very similar in chemical composition and general characters to the amphiboles (q.v.) . Although crystallizing in three different systems, they all possess distinct prismatic cleavages, the angles between which are about 87° (the cleavage angle in the amphiboles being 56°) . They are metasilicates, but, as shown in the following table, the composition varies widely in the different species, with corresponding differences in the various physical characters . The name pyroxene was originally given by R . J . Haiiy in 1796 to the black crystals of augite found in the lavas of Vesuvius and Etna: he derived the name from the Greek a6p (fire) and ivos (a stranger), because he thought that the crystals had been accidentally caught up by the lavas which contained them . As a matter of fact, the pyroxenes are, next to the felspars, the commonest constituents of igneous rocks of almost all kinds, being especially characteristic of those of basic composition . An igneous rock composed almost wholly of pyroxene is known as a pyroxenite . Besides being minerals of primary origin in igneous rocks, the pyroxenes are also of frequent occurrence in metamorphic rocks, for example, in crystalline limestones, being then of secondary origin . At the present day the name pyroxene is used as a group name for all the minerals enumerated below, though sometimes it is also applied as a specific name to include the monoclinic members diopside, hedenbergite, schefferite and augite . Orthorhombic Series . Enstatite MgSiOi .

Bronzite (Mg,Fe)SiOi . Hypersthene (Fe, Mg) SiOa . Monoclinic Series . Diopside CaMg(SiOa)2 . Hedenbergite CaFe(SiOa)z . Schefferite (Ca,Mg) (Fe,Mn) (SiOa)2 . Augite Ca(Mg,Fe)(SiO3)2with (Mg,Fe)(Al,Fe)2SiO6 . Acmite NaFe ";SiOa)2 . Spodumene LiAl(SiO3)2 . Jadeite NaAI(SiOa)2 . Wollastonite CaSiO3 . Pectolite HNaCa2(SiO3)a .

Rosenbuschite Na2Caa[(Si,Zr,Ti)Oa]a . Anorthic Series .

Rhodonite MnSiO3 . Babingtonite (Ca,Fe,Mn)SiO8.Fe2"'(SiO3)a . Hiortdahlite (Ca,Na)2F[(Si,Zr)02] . For details respecting the special characters and modes of occurrence of most of these species reference may be made to the respective headings: others not so treated are briefly mentioned below . Hedenbergite, or calcium iron pyroxene, is a black mineral closely allied to diopside (q.v.) and, owing to the isomorphous replacement of iron by magnesium, there is no sharp line of division between them . Schefferite, or manganese pyroxene, is a brown mineral found in the manganese mines of Sweden . Pectolite is a secondary mineral occurring as white masses with a radially fibrous structure in the veins and cavities of basic igneous rocks . Babingtonite is found as small black crystals on felspar in the granite of Baveno in Italy, and in the Haytor iron mine in Devonshire . Rosenbuschite, hiortdahlite, and some other rare members containing zirconium and fluorine, occur as accessory constituents in the nephelinesyenite of southern Norway .