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. Haüy in 1796 to the black crystals of augite found in the lavas of Vesuvius and Etna: he derived the name from the Greek πῦρ (fire) and ξένος (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 | MgSiO3. |
| Bronzite | (Mg, Fe)SiO3. |
| Hypersthene | (Fe, Mg)SiO3. |
| Monoclinic Series. | |
| Diopside | CaMg(SiO3)2. |
| Hedenbergite | CaFe(SiO3)2. |
| Schefferite | (Ca, Mg)(Fe, Mn)(SiO3)2 |
| Augite | Ca(Mg, Fe)(SiO3)2 with (Mg, Fe)(Al, Fe)2SiO6 |
| Acmite | NaFe‴(SiO3)2. |
| Spodumene | LiAl(SiO3)2 |
| Jadeite | NaAl(SiO3)2. |
| Wollastonite | CaSiO3 |
| Pectolite | HNaC32(SlO3)3. |
| Rosenbuschite | Na2Caa[(Si, Zr, Ti)O3]4. |
| Anorthic Series. | |
| Rhodonite | MnSiO3. |
| Babingtonite | (Ca, Fe, Mn)SiOa-Fe2‴(SiO3)3. |
| Hiortdahlite | (Ca, Na)2F[(Si, Zr)O2] |
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.