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Formation in cratons
junshan02 发表于 2012/8/1 14:42:00 |
The formation of natural diamond requires very specific conditions—exposure of carbon-bearing materials to high pressure, ranging approximately between 45 and 60 (4.5 and 6 ), but at a comparatively low temperature range between approximately 900–1300 °C. These conditions are met in two places on Earth; in the below relatively stable Natural history,Moncler Down
Vest Men and at the site of a meteorite strike.The conditions for diamond formation to happen in the lithospheric mantle occur at considerable depth corresponding to the requirements of temperature and pressure. These depths are estimated between 140 and 190 km though occasionally diamonds have crystallized at depths about 300 km as well. The rate at which into the Earth varies greatly in different parts of the Earth. In particular, under oceanic plates the temperature rises more quickly with depth, beyond the range required for diamond formation at the depth required. The correct combination of temperature and pressure is only found in the thick, ancient, and stable parts of continental plates where regions of lithosphere known as exist. Long residence in the cratonic lithosphere allows diamond crystals to grow larger.
Through studies of carbon ratios (similar to the methodology used in , except with the and ), it has been shown that the carbon found in diamonds comes from both inorganic and organic sources. Some diamonds, known as , are formed from inorganic carbon originally found deep in the Earth's mantle. In contrast, diamonds contain organic carbon from organic that has been pushed down from the surface of the Earth's through (see ) before transforming into diamond. These two different source of carbon have measurably different 13C:12C ratios. Diamonds that have come to the Earth's surface are generally quite old, ranging from under 1 to 3.3 billion years old. This is 22% to 73% of the age of the .Diamonds occur most often as or rounded and octahedra known as macles. As diamond's crystal structure has a cubic arrangement of the atoms, they have many that belong to a , octahedron, , or Moncler
Down Jackets Women. The crystals can have rounded off and unexpressive edges and can be elongated. Sometimes they are found grown together or form double "twinned" crystals at the surfaces of the octahedron. These different shapes and habits of some diamonds result from differing external circumstances. Diamonds (especially those with rounded crystal faces) are commonly found coated in nyf, an opaque gum-like skin. |
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