The diamonds are delivered to the surface in pieces of rock, known as xenoliths, which are torn from the mantle by deep-source volcanic eruptions. When the mantle material approaches the surface, an explosive eruption occurs that forms a pipe-shaped structure that might be several hundred yards to over a mile in diameter. These "pipes," the rocks that are blasted from them, and the sediments and soils produced by their weathering are the source for most of Earth's natural diamonds.
The best way to learn about rocks is to have specimens available for testing and examination. Some peridotites contain significant amounts of chromite. Some of these form when a subsurface magma slowly crystallizes. During the early stages of crystallization, the highest-temperature minerals such as olivine, orthopyroxene, clinopyroxene, and chromite begin to crystallize from the melt.
The crystals are heavier than the melt and sink to the bottom of the melt. These high-temperature minerals can form layers of peridotite on the bottom of the magma body. These are known as "stratiform deposits. Another type of chromite deposit occurs where tectonic forces push large masses of oceanic lithosphere up onto a continental plate in a structure that is known as an "ophiolite.
Aeromagnetic prospecting: Finding small bodies of peridotite such as a kimberlite pipe can be very difficult because they are so small. Aeromagnetic surveys are sometimes employed to find them. The geographic areas underlain by peridotite will often be a magnetic anomaly in contrast to their surrounding rocks. Images by the United States Geological Survey. Peridotite bodies exposed at Earth's surface are rapidly attacked by weathering.
They can then be obscured by soil, sediment, glacial till, and vegetation. Finding a peridotite body as small as a kimberlite pipe, which might be only a few hundred yards across, can be very difficult.
Because peridotite often has magnetic properties that are distinctly different from the surrounding rocks, a magnetic survey can sometimes be used to locate them. The survey can be conducted using an aircraft that slowly tows a magnetometer at low altitudes, recording the magnetic intensity as it travels.
The magnetic data can be plotted on a map, often revealing the location of the pipe as an anomaly. See map and photo. Peridotite bodies are also found by prospecting for some of the rare minerals that they contain.
When a peridotite weathers, the olivine breaks down, quickly leaving the more resistant minerals behind. Geologists have located peridotite bodies by prospecting for chromite, garnet, and other resistant indicator minerals. When scattered by the action of water, wind, or ice, they will be most highly concentrated near the pipe and be diluted by local rock debris with distance.
The grains of these minerals might also be more rounded with distance of transport. This allows geologists to use the "trail-to-lode" prospecting method to find them. They sometimes contain chromite or diamonds. Article by: Hobart M. Schulte, Ryan D. Taylor, Nadine M. Piatak, and Robert R. Serpentinite is used as a decorative stone because of its interesting texture.
Nickel and platinum are usually associated with ultramafic host rocks. Dunite xenolith in basaltic lava from Hawaii. Width of sample 8 cm. A sample from the Troodos ophiolite in Cyprus with a weathering rinds.
Unaltered rock is in the middle. Width of sample 11 cm. Peridotite with a huge garnet crystal. Hullvann, Norway. Width of sample 18 cm. Harzburgite orthopyroxene peridotite from the Seiland magma province in Norway. Width of sample 10 cm. Harzburgite from the Troodos ophiolite.
This is example of a depleted mantle. Layered harzburgite from the Troodos ophiolite. Serpentinite sample from the Troodos ophiolite. It is a hydrothermally altered peridotitic rock. This is a sample of dunite from a working quarry. Dunite is mined because of its high olivine content.
Olivine can be used as a refractory material. Gusdal quarry, Norway. Width of sample 9 cm. Main minerals are pyrope, chromian diopside and olivine. Width of view 25 cm.
Pyroxenite layers in a layered peridotite intrusion. Lower pyroxenite layer is about 5 cm thick. Closer view of a pyroxenite layer in pyrope-bearing dunite. And more distant look at the same outcrop. Another sample from Norway showing pyrope and diopside phenocrysts in a finer groundmass which is mostly composed of olivine.
Peridotite with abundant alteration patches of dark green chlorite. Helgehornsvatnet, Norway. Le Maitre, R. Dunite xenolith in basaltic lava from Hawaii. Width of sample 8 cm. From Sand Atlas. From James St. Wherlite with pyrope red and Cr-diopside green. Garnet lherzolite: xenolith from a kimberlite pipe, Kimberley, central South Africa. Verde Sao Vicente C. Cumulate Adcumulate Orthocumulate.
Larvik complex Ekerite Larvikite. Oka complex Niocalite carbonatite Monticellite carbonatite. Accessory phases include garnet, spinel, plagioclase, ilmenite, chromite and magnetite.
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