Pyroxene

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 irvp (fire) and vos (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. MgS103.

Bronzite. (Mg,Fe)S103.

Hypersthene. .. (Fe,Mg)S103. Monoclinic Series.

Diopside CaMg(S103)2.

Hedenbergite CaFe(S103)2.

Schefferite (Ca, Mg) (Fe, M n) (S103)2.

A Ca(Mg,Fe)(S103)2with (Mg,Fe) (A1,Fe)2S106.

NaFe"(S103)2.

LiAl (S103)2.

essentially of hornblende. The term perknite (Gr. 7rEpuvos, dark) has also been used to designate the whole series.

They are essentially of igneous origin, though some pyroxenites are included in the metamorphic complex of the Lewisian of Scotland; those pyroxene rocks which result from the contact alteration of impure limestones are described as pyroxene hornfelses (calc-silicate hornfelses). The pyroxenites are closely allied to the gabbros and norites, from which they differ by the absence of felspar, and to the peridotites, which are distinguished from them by containing olivine. This connexion is indicated also by their mode of occurrence, for they usually accompany masses of gabbro and peridotite and seldom are found by themselves. They are strictly plutonic and often very coarse-grained, containing individual crystals which may be several inches in length. The principal accessory minerals, in addition to olivine and felspar, are chromite and spinels, garnet, iron oxides, rutile, scapolite. They frequently occur in the form of dikes or segregations in gabbro and peridotite: e.g. in Shetland, Cortlandt on the Hudson river, North Carolina (websterite), Baltimore, New Zealand, and in Saxony. The component minerals often have a close resemblance to those of the surrounding rock. By decomposition the rocks consisting of pyroxene pass into serpentines, which sometimes preserve the original structures of the primary minerals, such as the lamination of hypersthene and the rectangular cleavage of augite. Under pressure-metamorphism hornblende is developed and various types of amphibolite and hornblende-schist are produced. Occasionally rocks rich in pyroxene are found as basic facies of nepheline syenite; a good example is provided by the melanite pyroxenites associated with borolanite at Ledbeg in Sutherlandshire.

(J. S. F.)