Pyroxenite

PYROXENITE, a rock consisting essentially of minerals of the pyroxene group, such as augite and diallage, hypersthene, bronzite or enstatite. Names have been given to members of this group according to their component minerals, e.g. pyroxenite (augite), diallagite (diallage), hypersthenite (hypersthene), bronzitites (bronzite), websterite (diallage and hypersthene). Closely allied to this group are the hornblendites, consisting Crystals have the form of hexagonal plates bounded at their edges by faces of a hexagonal prism and pyramids, which are deeply striated horizontally. More frequently, however, the mineral is massive, with a laminar or granular structure. The colour is bronzeyellow and the lustre metallic; the streak is greyish-black. The hardness is 4 and the specific gravity 4'58-4.64. The mineral is magnetic, sometimes with polarity, and it is therefore often called "magnetic pyrites." Pyrrhotite occurs in metalliferous veins, and as grains and plates disseminated through various rocks. In the gabbros and norites of Norway and Sweden it has been concentrated by magmatic differentiation at the margins of the igneous masses. Large bodies of massive pyrrhotite occur at Bodenmais in Bavaria and in Wheal Jane near Truro in Cornwall. Crystallized specimens are from the metalliferous veins at Morro Velho in Brazil, Kongsberg in Norway, and Andreasberg in the Harz. Crystals of pyrrhotite have also been observed in meteoric stones; but iron sulphide appears more commonly in meteorites, especially in meteoric irons, as troilite (FeS), which, if really distinct from pyrrhotite, has not been met with in terrestrial rocks. (L. J. S.)

Categories: PRU-PYT | Petrology