Peridotite (harzburgite) from the Precambrian of Montana, USA. Igneous rocks form by the cooling &amp; crystallization of hot, molten rock (magma &amp; 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. Peridotites are coarsely-crystalline, ultramafic, intrusive igneous rocks. Igneous rocks have an ultramafic chemistry if they are &lt;45% silica (= SiO2 chemistry), are dominated by the mineral olivine, and often have some pyroxene. They are often greenish-colored, greenish-black, or brownish-black. Peridotites 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 peridotites (or metamorphosed peridotites) include Oman, Cyprus, the Shetland Islands, Austria, and Newfoundland. Sometimes, peridotite 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 peridotite xenoliths. There are four principal varieties of peridotite, based on different percentages of the three main minerals: olivine, clinopyroxene, and orthopyroxene. These four rocks are dunite, harzburgite (formerly saxonite), wehrlite, and lherzolite. Dunite has 90 to 100% olivine, with minor pyroxene. Lherzolite, harzburgite, and wehrlite have decent mixes of olivine &amp; pyroxene. Harzburgite is a mix of olivine and orthopyroxene (40 to 90% olivine &amp; 10 to 60% orthopyroxene). Wehrlite is a mix of olivine and clinopyroxene (40 to 90% olivine &amp; 10 to 60% clinopyroxene). Lherzolite is a mix of olivine (40 to 90% of the rock), plus orthopyroxene and clinopyroxene. The peridotite shown above is a harzburgite, having olivine and orthopyroxene. It comes 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
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
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<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/16601144800/">Harzburgite (Stillwater Complex, Neoarchean, 2.71 Ga; Beartooth Mountains, Montana, USA)</a>