sexta-feira, 25 de abril de 2008

THE WORLD OF DIAMONDS

Diamond is carbon in its most concentrated form. Except for trace impurities like boron and nitrogen, diamond is composed solely of carbon, the chemical element that is fundamental to all life. But diamond is distinctly different from its close cousins the common mineral graphite and lonsdaleite, both of which are also composed of carbon. Why is diamond the hardest surface known while graphite is exceedingly soft? Why is diamond transparent while graphite is opaque and metallic black? What is it that makes diamond so unique?
The key to these questions lie in diamond's particular arrangement of carbon atoms or its crystal structure--the feature that defines any mineral's fundamental properties. A crystal is a solid body formed from the bonding of atomic elements or compounds in a repeating arrangement. Often, crystals possess smooth external faces. Due to their symmetrical and finite nature, the building blocks of crystals are limited to relatively small numbers of atoms, and their chemical composition.

This exhibition appeared at the Museum November 1, 1997 – August 30, 1998.It will be on display at the Midland Center for the Arts July 12 – November 3, 2002 -DIAMOND.
The mere mention of the word fills the mind with a multitude of concepts and images. Diamond is a mineral, a natural crystalline substance, the transparent form of pure carbon. Diamond is something superb, the peerless "king of gems" that glitters, dazzles, and symbolizes purity and strength. Diamond is for engagement and the 75th wedding anniversary, for a commitment to never-ending love. Diamond is indomitable, the hardest surface known. 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, fully two thirds the age of the Earth. Diamond is a strategic and high-tech supermaterial for our technological society. Diamond is a shape. This exhibition presents the fascinating story of the nature of diamonds. compositions to simple numerical combinations of elements.
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, low- temperature, 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.
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.

The map above shows both the major deposits and the ancient bedrock, both the 2,500-million-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.

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