File:Pyroxenite (Stillwater Complex, Neoarchean, 2.71 Ga; Beartooth Mountains, Montana, USA) (16762707716).jpg
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Pyroxenite from the Precambrian of Montana, USA. Igneous rocks form by the cooling & crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy. Pyroxenites are coarsely-crystalline, ultramafic, intrusive igneous rocks. Igneous rocks have an ultramafic chemistry if they are <45% silica (= SiO2 chemistry) and are dominated by the minerals pyroxene and/or olivine. They are often greenish-black, greenish-brown, or brownish-black. Pyroxenites are scarce in much of the Earth’s crust, but are common in Earth's upper mantle. There are a few places on Earth where mantle rocks have been uplifted to the surface. Examples of localities with exposed mantle pyroxenites (or metamorphosed pyroxenites) include Oman, Cyprus, the Shetland Islands, Austria, and Newfoundland. Sometimes, pyroxenite rocks from the mantle get caught up in rising masses of magma. When erupted from volcanoes at the surface, the lava flows will have mantle pyroxenite xenoliths. There are six principal varieties of pyroxenite, based on different percentages of the three main minerals: orthopyroxene, clinopyroxene, and olivine. These six rocks are orthopyroxenite, clinopyroxenite, websterite, olivine websterite, olivine orthopyroxenite, and olivine clinopyroxenite. The first three named rocks are essentially pyroxene-only igneous rocks, with little to no olivine content. Websterite has a mix of orthopyroxene and clinopyoxene. The latter 3 named rocks have 10 to 40% olivine content mixed with the pyroxene. The rock shown above is a pyroxenite from a famous large layered igneous intrusion in Montana - the Stillwater Complex, where platinum and palladium are mined. Stratigraphy: Stillwater Complex, Neoarchean, 2.71 Ga Locality: Beartooth Mountains, southern Montana, USA |
| Date | |
| Source | Pyroxenite (Stillwater Complex, Neoarchean, 2.71 Ga; Beartooth Mountains, Montana, USA) |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/16762707716 (archive). It was reviewed on 5 December 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| Date/Time | Thumbnail | Dimensions | User | Comment | |
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| current | 05:45, 5 December 2019 | | 967 × 846 (1.4 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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