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NATURE OF DIAMOND http://www.amnh.org/exhibitions/diamonds/ Diamond is exotic, formed in Earth's interior and shot to the surface by extraordinary volcanoes. A diamond is likely the oldest thing you will ever own, probably 3 billion years in age, two thirds the age of the Earth. Diamond is a strategic and high-tech supermaterial for our technological society. Diamond is amazingly dense. At 3.51 grams per cubic centimeter, it is vastly more dense than graphite -- the more common form of the light element carbon -- at 2.20 grams per cubic centimeter. This comparison offers an important clue to diamond's origin: The fact that diamond is "squeezed" much denser than graphite, which forms near Earth's surface, implies formation at high pressure. As shown on the graph, this concept was corroborated by experimental synthesis of diamond at high pressure and temperature. This simplified diagram shows the conditions of pressure and temperature where diamond and graphite will be the stable forms of carbon. The points show the conditions at which diamonds were first grown by the companies ASEA and General Electric in the early 1950s. Temperatures are in Kelvin; subtract 273 to convert to degrees Celsius. This magnitude of high pressure is difficult to comprehend. For example, to make a diamond at 1400 degrees C (orange hot) would require the pressure of 55,000 atmospheres! Experiments and the high density of diamonds tell us that they crystallize at very high pressures. In nature this means that diamonds are created by geologic processes at great depth within Earth, generally more than 150 kilometers down, in a region beneath the crust known as the mantle. Other processes, explored later in this exhibition, bring diamonds to where people can find them. This diagram shows the interior structure of Earth. The three concentric layers -- the core, mantle, and crust -- formed within a few hundred million years of Earth's coalescence 4.5 billion years ago. The core is primarily an iron-nickel alloy and makes up a large fraction of the mass of Earth. The vast mantle is sandwiched between the core and the thin crust and is composed predominantly of magnesium and iron silicate minerals. Our planet's crust is a thin, rocky skin. Diamonds can form in most of Earth's interior but not near its surface, where graphite is the stable form of carbon. Indeed, diamonds only survive at Earth's surface because great heat is required to break down the diamond structure. The upper mantle is slightly plastic, which allows it to circulate slowly in a creeping, convective flow that helps drive the surface motion of Earth known as "plate tectonics." The cross section shown here provides a closer look at Earth's crust and underlying mantle. The crust can be divided into ocean basins, underlain by a thin layer of dense, basaltic rock, and continents, formed of a much thicker but less-dense layer of granitic rocks. Just below the crust is the portion of the mantle called the lithosphere, which is rigid and acts like rock. Below this is the asthenosphere, a more plastic, flowing region that enables the overlying crustal plates to move in what is known as plate tectonics. The plot of pressure and temperature shows the conditions at which either diamond or graphite exist. The general conditions present in the Earth are described by curved lines called geotherms. Note that there are two geotherms: Because the continental crust is old and thick, conditions are somewhat colder in and beneath it than beneath the much younger ocean basins. Diamonds can form at depths as shallow as 150 kilometers beneath the continental crust, while beneath oceans they need depths of at least 200 kilometers, as shown by the diamond boundary on the cross-section. The search for diamonds has determined that most are derived from kimberlite pipes in the oldest, nuclear portions of the continents, where the basement rocks are older than 1.5 billion years. The oldest parts of continents are called cratons, and can be divided into two terranes: Archean-age archons, which are older than 2,500 million years, and Proterozoic-age protons, which are 1,600 -- 2,500 million years old. The distribution of these terranes is shown on the map. Kimberlite pipes occur in many parts of the continental crust, but most diamond-rich ones are found in archons. This fact suggests that most diamonds were formed and stored deep below the cratons, in the area shown in the lower figure, and were later transported to the surface by kimberlite and lamproite magmas that extracted them and other samples from the mantle. The complex volcanic magmas that solidify into kimberlite and lamproite are not the source of diamonds, only the elevators that bring them with other minerals and mantle rocks to Earth's surface. Although rising from much greater depths than other magmas, these pipes and volcanic cones are relatively small and rare, but they erupt in extraordinary supersonic explosions. Kimberlite and lamproite are similar mixtures of rock material. Their important constituents include fragments of rock from Earth's mantle, large crystals, and the crystallized magma that glues the mixture together. The magmas are very rich in magnesium and volatile compounds such as water and carbon dioxide. As the volatiles dissolved in the magma change to gas near Earth's surface, explosive eruptions create the characteristic carrot- or bowl-shaped pipes. Kimberlite magma rises through Earth's crust in networks of cracks or dikes. The pipes only form near Earth's surface. This cross-section of a kimberlite pipe shows the carrot-shaped profile produced by explosive eruption. The root zone starts in fissures, where gases are released from the rising magma and drive the eruption; they blow out the fragment-laden kimberlite to form the volcano's tuff ring and fill the pipe. Depth measurements show the level of erosion for various kimberlite pipes in South Africa. Adapted from Hawthorne (1975). These drawings illustrate the formation and filling of the typical champagne-glass shape of a lamproite pipe. The initial stage of the eruption, powered by gases either from the lamproite magma or from boiling ground water, corrodes the hosting rock to form the champagne-glass shape (top). The eruption then produces particles of ash, lapilli, and pumice that partially fill the crater and form a tuff ring (middle). Finally, the crater fills with a lava pond from the degassed lamproite magma (bottom). Adapted from a sketch by Barbara Scott-Smith Kimberlite magmas carry foreign rocks -- xenoliths -- from Earth's mantle to the surface. Xenoliths are geologists' only samples from the deep Earth, and carry information about diamond growth conditions. The 2 most common types of xenoliths are peridotites and eclogites. Peridotite is the main constituent of the mantle beneath the crust and consists primarily of olivine -- the gem variety is peridot. Eclogite, consisting primarily of garnet and a green pyroxene, is formed by plate tectonics when basalt of the ocean crust founders into the mantle. Certain kinds of xenoliths contain diamonds. These diagrams show the compositions of mantle xenoliths. Lherzolite is a variety of peridotite thought to form most of the upper mantle. Harzburgite is another kind of peridotite with less clinopyroxene. Garnet harzburgites contain red garnet and, occasionally, diamonds. Eclogite, a very different rock, consists of garnet and sodium-rich pyroxene; some also contains diamonds. Diamonds with inclusions are like little space capsules from the mantle: pristine mineral samples are protected by the diamond's indomitable embrace and transported to the surface by a volcanic rocket. Inclusions capture a picture of the rock and environment in which diamonds grow and indicate that garnet harzburgite (a type of peridotite) and eclogite are the most common rocks in which diamonds have grown. A single mineral inclusion rarely defines a specific rock, but two or more minerals may enable interpretation of rock associations and origin. Some inclusion minerals are virtually unique to diamond sources and are thus sought in the exploration for diamonds. A purple pyrope garnet (a.), an indicator of garnet harzburgite, in a brownish diamond octahedron from the Udachnaya pipe, Sakha Republic, Russia (about 0.8 mm across). Orange "G5" garnet (b.), typical of diamond eclogite, showing the conspicuous octahedral shape imposed by the enclosing diamond (about 0.5 mm across). Red chromian pyrope and green chromian diopside, indicators of a peridotite, in a diamond octahedron from the Mir pipe, Sakha Republic, Russia (each about 0.2 mm across). Most diamonds consist of primeval carbon from Earth's mantle, but those from eclogites probably contain carbon recycled from the ocean crust by plate tectonics -the carbon of microorganisms. How do we know? Carbon atoms occur in three different masses, or isotopes. Unlike high-temperature processes in deep Earth, lowtemperature, biological processes, such as photosynthesis, are sensitive to the differences in mass, and actively sort different carbon isotopes. Thus, the ratios of carbon isotopes in organic materials -- plants, animals, and shells -- vary, and also differ from those in the carbon dioxide of the atmosphere and the oceans. Geochemists "read" the carbon isotopes in samples to interpret nature's record. Virtually all carbon atoms, the ones in a diamond or a tree or you, came from the stars. Particularly at Earth's surface the proportions of 12C and 13C (the carbon isotopes of mass 12 and 13) get redistributed. Expressed as simple numbers in 13C notation -- in which larger numbers mean more 13C -- organic carbon has large negative values, average Earth has a mildly negative value, and the carbon in shells is near zero. The narrow range of 13C values for harzburgitic diamonds in the histogram on the top resembles the range of average Earth, indicating that the mantle is the likely carbon source. The large range for eclogites suggests mixing of organic carbon (the strongly negative numbers), mantle carbon (mildly negative numbers), and shell-like carbon (values near zero). These data support recycling of once-living carbon from Earth's surface deep into the mantle to form diamond. When ocean floor slides into the mantle, the basaltic rock becomes eclogite, and organic carbon in sediments may become diamond. Kimberlites are generally much younger than the diamonds they bring to Earth's surface. Kimberlites and lamproites have been dated between 50 and 1,600 million years old. Diamonds associated with harzburgites are about 3.3 billion years old -more than two thirds the age of Earth itself, and those from eclogites generally range from 3 billion to less than 1 billion years old. These age differences help clarify a picture of diamonds having crystallized and been stored beneath the ancient continental cratons and only later being lifted to Earth's surface by kimberlites. Since inclusion minerals crystallized simultaneously with their diamond host, the age of the inclusions gives the age of the diamond. The ancient age of peridotite diamonds suggests that the formation of ancient Archean continental cores (archons) included diamond crystallization in the underlying mantle lithosphere. A relatively cool, rigid, deep keel beneath these continental nuclei provided a stable environment in which diamonds crystallized and were stored. Subsequently, oceanic crust diving into the mantle was metamorphosed into eclogite and pasted onto this keel. Much later passage of kimberlite magmas through the keel dislodged diamonds from both peridotite and eclogite and sent them to Earth's surface. This cross-section of continental crust shows the 200-km-thick cool keel (part of the mantle lithosphere) that provided a stable environment for diamond crystallization and preservation. Kimberlites centered over the keel are likely to yield harzburgitehosted diamonds from the storage zone (marked with diamonds). Kimberlites near the edge of the keel are more likely to contain eclogite-hosted diamonds, while those off the keel are likely to be barren of diamonds. In the last 20 years scientists have discovered new sources of diamond. Continental collisions -- a result of plate tectonics -- can subject slices of a crust to immense burial and uplift. In Kazakhstan, for example, diamonds formed in buried crust that returned to Earth's surface. Meteor impacts produce immense pressures, and diamonds can be formed and sprayed among the impact debris. Meteorites also experience impacts themselves and can contain diamonds. And the most ancient meteorite material contains star dust, the remnants of the death of stars. Some of this star dust is extremely tiny bits of diamond, just big enough to be crystals and older than the solar system itself. Very small "microdiamonds," averaging only 12 micrometers across, were discovered during diamond exploration in a region called the Kokchetav Massif, in northern Kazakhstan, in large slices of metamorphic rock that must have been pushed at least 120 kilometers deep into Earth and returned. Discovery of this process, termed ultrahigh pressure (UHP) metamorphism, has revolutionized ideas about and interest in what can happen to Earth's crust. Recently scientists have found traces of diamond around meteor impacts. At the 35-million-year-old Popigai crater in Siberia, graphite transformed into microdiamond aggregates up to 1 centimeter across. It is now suspected that diamonds form in most major impacts, becoming a new indicator of ancient cosmic collisions. In 1987, microscopically small fragments of diamond, called "nanodiamonds," were recovered from meteorites that predate the solar system. New studies indicate that they formed more than 5 billion years ago in flashes of radiation from dying red-giant stars into surrounding clouds of methane-rich gas. The process is essentially the same as the new process for growing synthetic diamond called CVD -- chemical vapor deposition. Cartoon of the formation of a UHP terrane that can yield diamonds. At top, the down-going subducted ocean crust (green) has a thin covering of sediment (gray) that is sheared off and driven upward (inset), apparently caused by the continental collision (middle) that squeezes the diamond-bearing metamorphic rocks back into the crust (bottom). Certain minerals are present in the rocks from the upper mantle that occur with diamonds in kimberlite and lamproite pipes, as seen in nearby cases of xenoliths and diamond inclusions. Some of these minerals, being resistant to weathering and denser than quartz sand, concentrate in channel bottoms. Because they occur in far greater abundance than diamond, exploration geologists look for these "indicators" among the gravel of regions they suspect may host diamond-bearing pipes. Indicator minerals for diamond include, in order of decreasing significance: garnet, chromite, ilmenite, clinopyroxene, olivine, and zircon. But the order of persistence in streams is zircon, ilmenite, chromite, garnet, chromian diopside, and olivine. Diamond itself is obviously a most important indicator. Most indicator minerals have a distinctive color. Seen here are red pyrope garnets, green chromian clinopyroxene, black ilmenite and chromite, and yellowish-green olivine. The best way to see a kimberlite pipe is first hand, like a miner or geologist, in the tunnels that provide access to the pipe in an underground mine. The tunnel recreated in the exhibition goes from the local bedrock, through a boundary zone that is highly fragmented, and into the kimberlite, with its inclusions of mantle rocks and diamonds. Today diamonds are mined in about 25 countries, on every continent but Europe and Antarctica. However, only a few diamond deposits were known until the 20th century, when scientific understanding and technology extended diamond exploration and mining around the globe. For 1,000 years, starting in roughly the 4th century BCE, India was the only source of diamonds. In 1725, important sources were discovered in Brazil, and in the 1870s major finds in South Africa marked a dramatic increase in the diamond supply. Additional major producers now include several African countries, Siberian Russia, and Australia. It is a modern misconception that the world's diamonds come primarily from South Africa: diamonds are a world-wide resource. The common characteristic of primary diamond deposits is the ancient terrain that hosts the kimberlite and lamproite pipes that bring diamonds to Earth's surface. This map shows both the major deposits and the ancient bedrock, both the 2,500million-year-old archons and less productive 1,600 to 2,500-million-year-old protons, that contain the diamond pipes. The diamonds in secondary deposits have been moved by erosion away from the pipes. The monumental increase in diamond production in the 20th century is shown on this graph. India's maximum production, perhaps 50,000 to 100,000 carats annually in the 16th century, is very small by modern standards. Brazil and Venezuela are barely discernible compared to South African production following discoveries in 1867. For the most part, except for major wars and economic recessions, diamond production has been steadily increasing since then, with non-African sources growing in relative proportion. Major production is now dominated by Australia, Botswana, Russia, and Congo Republic (Zaire), but South Africa is still a major producer, in both volume and value. Eighty percent of the diamonds mined annually are used in industry; 4 times that production is grown synthetically for industry - that's a total of over 500 million carats or 100 metric tons. Diamond is a fundamental industrial material that affects our daily lives. Because diamond is the hardest substance, it is used to cut, grind, and polish most hard substances. It fashions stones, ceramics, metals, and concrete, as well as eyeglasses, gems, and computer chips. Its growing specialty-uses include blades, some used in critical surgery; specialty windows; and heat spreaders. And of course diamond phonograph needles reproduced music for 50 years. Diamond has three primary roles in industry: it is used as a cutting tool, it is imbedded in another material and used as a tool or abrasive, and it is turned to powder or paste for grinding and polishing. Diamond is selected for such use where its hardness and resistance to abrasion - its long working life and fast cutting action - outweigh its costs. Moreover, diamond's resistance to wear enables it to cut reproducibly time after time, a requirement of automated production. Diamond machining tools for turning, milling, and boring are preferred where finely finished surfaces of high precision are needed. Diamond is used for machining a wide variety of plastics, glasses, and metals, shaping products such as the drums for copying machines, polygon mirrors in laser printers, and aluminum-alloy pistons in automobile engines. However, diamond cannot be used for machining alloys of iron. Under intense machining conditions the diamond abrades very quickly against some materials, apparently because of a hightemperature reaction between iron and carbon to yield iron carbide. ------------------------