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The Devonian extinction crisis primarily
junshan02 发表于 2012/8/1 14:38:00 |
The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems .Amongst the severely affected marine groups were the Moncler Down
Vest Men, trilobites, , , and , as well as jawless fish, and all placoderms. Land plants as well as freshwater species, such as our tetrapod ancestors, were relatively unaffected by the Late Devonian extinction event.The reasons for the Late Devonian extinctions are still unknown, and all explanations remain speculative. suggested in 1969 that the Devonian extinction events were caused by an asteroid impact. However, while there were Late Devonian collision events (see the ), little evidence supports the existence of a Devonian crater large enough.
In , diamond (from the ancient αδ?μα? – adámas "unbreakable") is an , where the carbon are arranged in a variation of the crystal structure called a . Diamond is than , but the conversion rate from diamond to graphite is negligible at . Diamond is renowned as a material with superlative physical qualities, most of which originate from the strong between its atoms. In particular, diamond has the highest and of any bulk material. Those properties determine the major industrial application of diamond in cutting and polishing tools and the scientific applications in and .Diamond has remarkable optical characteristics. Because of its extremely rigid lattice, it can be contaminated by very few types of impurities, such as and Moncler Jackets. Combined with wide transparency, this results in the clear, colorless appearance of most natural diamonds. Small amounts of defects or impurities (about one per million of lattice atoms) color diamond blue (boron), yellow (nitrogen), brown (), green (radiation exposure), purple, pink, orange or red. Diamond also has relatively high (ability to disperse light of different colors), which results in its characteristic . Excellent optical and mechanical properties, combined with efficient marketing, make diamond the most popular . |
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