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Pyroxenite

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Pyroxenite is a plutonic rock that consists of dark minerals in the pyroxene group plus a little olivine or amphibole minerals. (more below)
Black deep seafloor
Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)
Pyroxenite belongs to the ultramafic group, meaning that it consists almost entirely of dark minerals rich in iron and magnesium. Specifically, its silicate minerals are mostly pyroxenes rather than the other mafic minerals, olivine and amphibole. In the field, pyroxene crystals display a stubby shape and square cross-section whereas amphiboles have a lozenge-shaped cross-section.

Pyroxenite is often associated with its ultramafic cousin peridotite. Rocks like these originate deep in the seafloor, underneath the basalt that makes up the upper oceanic crust. They occur on land where slabs of oceanic crust become attached to continents, that is, at subduction zones.

Identifying this specimen, from the Feather River Ultramafics of the Sierra Nevada, was largely a process of elimination. It attracts a magnet, probably due to fine-grained magnetite, but the visible minerals are translucent with a strong cleavage. I was reliably informed that the locality contained ultramafics. Greenish olivine and black hornblende are absent, and the hardness of 5.5 also ruled out these minerals as well as the feldspars. Without large crystals, a blowpipe and chemicals for simple lab tests, or the ability to make thin sections, this is as far as the amateur can go sometimes.

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