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Diamonds and Peridotite
The formation of diamonds requires very high temperatures and pressures that only occur on Earth at depths of 100 miles below the surface and at locations in the mantle where temperatures are at least 2000 degrees Fahrenheit. The diamonds are delivered to the surface in pieces of rock, known as xenoliths, which are torn from…
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Ophiolites, Pipes, Dikes and Sills
Earth’s mantle is thought to be composed mainly of peridotite. Some of the occurrences of peridotite on Earth’s surface are thought to be rocks from the mantle that have been brought up from depth by deep-source magmas. Ophiolites and pipes are two structures that have brought mantle peridotite to the surface. Peridotite is also found…
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Alteration of Peridotite
Peridotite is a rock type that is more representative of Earth’s mantle than of the crust. The minerals that compose it are generally high-temperature minerals that are unstable at Earth’s surface. They are quickly altered by hydrothermal solutions and weathering. Those that contain magnesium-oxide-bearing minerals can alter to form carbonates, such as magnesite or calcite, which are much…
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Many Types of Peridotite
The peridotite “family” contains a number of different intrusive igneous rocks. These include lherzolite, harzburgite, dunite, wehrlite, and kimberlite (see photos). Most of them have an obvious green color, attributed to their olivine content.
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What is Peridotite?
Peridotite is a generic name used for coarse-grained, dark-colored, ultramafic igneous rocks. Peridotites usually contain olivine as their primary mineral, frequently with other mafic minerals such as pyroxenes and amphiboles. Their silica content is low compared to other igneous rocks, and they contain very little quartz and feldspar. Peridotites are economically important rocks because they often contain chromite – the only ore of chromium; they can be source rocks for diamonds;…
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A Confusion of Names
Flint is a microcrystalline variety of quartz. Materials of this description have been given a wide variety of names, including chert, jasper, agate, and chalcedony. Most geologists use the word “chert” instead of “flint”. Some people believe that the name “flint” should be reserved for dark-colored chert that formed as nodules in limestone or chalk.…
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Flint as a Construction Material
Where flint is abundant it is sometimes used as a construction material. It is very durable and resists weathering better than almost any other natural stone. It is common to see walls, homes, and larger buildings that are built partially or entirely with flint as a facing stone in southern England and many parts of Europe.
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Flint as a Gemstone
Flint is a very durable material that accepts a bright polish and often occurs in attractive colors. It is occasionally cut into cabochons, beads, and baroque shapes for use as a gemstone. It is also used to produce tumbled stones in a rock tumbler. Most people have heard of a gem material called “jasper”. Jasper is an opaque variety…
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Flint as a Source of Fire
Another important property of flint is its ability to generate sparks of hot material when it is struck against steel. This property allows flint to be used as a fire-starter. Skilled people can use a piece of flint, a piece of steel, and a little tinder to quickly start a fire. Early firearms, such as…
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Neolithic Flint Miners
Perhaps the most impressive story about flint is that of the ancient mining complexes that were built in what is now England during Neolithic times. These excavations began about 4000 BC and continued until the widespread use of metals about 2,000 years later. One flint mining complex of particular note was Grime’s Graves located near Brandon, England.…