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Diamonds are almost impossible to detect directly because they are so rare: very rich kimberlite pipes（管子，烟斗）, the routes through which diamonds rise, may contain only three carats of diamonds per ton of kimberlite.Kimberlite begins as magma（岩浆） in Earth’s mantle (the layer between the crust and the core).As the magma smashes through layers of rock, it rips out（狠狠的发出） debris（碎片）, creating a mix of liquid and solid material. Some of the solid material it brings up may come from a so-calleddiamond-stability field, where conditions of pressure and temperature are conducive （有助的，有益的）to the formation of diamonds.If diamonds are to survive, though, they must shoot toward Earth’s surface quickly.Otherwise, they revert（恢复，还原） to graphite or burn. Explorers seeking diamonds look for specks（眼镜） of “indicator minerals” peculiar （奇怪的，奇异的）to the mantle but carried up in greater quantities than diamonds and eroded out（侵蚀） of kimberlite pipes into the surrounding land.The standard ones are garnets, chromites, and ilmenites.One can spend years searching for indicators and tracing them back to the pipes that are their source; however, 90 percent of kimberlite pipes found this way are barren of diamonds, and the rest are usually too sparse（稀少的） to mine.
In the 1970’s the process of locating profitable pipes was refined（优雅的，精炼的）by focusing on the subtle （微妙的）differences between the chemical signatures of indicator minerals（矿产品）found in diamond-rich pipes as opposed to those found in barren （贫瘠的）pipes.For example, G10 garnets, a type of garnet typically found in diamond-rich pipes, are lower in calcium （钙）and higher in chrome（铬） than garnets from barren pipes. Geochemists John Gurney showed that garnets with this composition（成分） were formed only in the diamond-stability field; more commonly found versions came from elsewhere in the mantle.Gurney also found that though ilmenites did not form in the diamond-stability field, there was a link useful for prospectors:when the iron in ilmenite was highly oxidized, its source pipe rarely contained any diamonds.He reasoned that iron took on more or less oxygen in response to conditions in the kimberlitic magma itself—mainly in response to heat and the available oxygen.When iron became highly oxidized, so did diamonds; that is, they vaporized （蒸发）into carbon dioxide.
GWD6-Q26:
According to the passage, Gurney refined （精炼的）the use of ilmenites in prospecting for diamonds in which of the following ways?
A.He found that ilmenites are brought up from the mantle by kimberlite pipes and erode out into the surrounding land in greater quantities than diamonds.
B.He found that since ilmenites do not form in the diamond-stability field, their presence indicates the absence of diamonds.
C.He showed that highly oxidized iron content in ilmenites indicates a low survival rate for diamonds.
D.He found that when the iron in ilmenites is highly oxidized, conditions in the magma were probably conducive to the formation of diamonds.
E.He showed that ilmenites take on more or less oxygen in the kimberlite pipe depending on the concentration of diamonds.
答案是C，想问问B错哪了？文中不是说Gurney also found that though ilmenites did not form in the diamond-stability field, there was a link useful for prospectors.。。。迷茫了。。。P.s:不爱好意思，本人单词量太有限，就查了那么多，见笑了~~嘿嘿 |
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发表于 2011-3-6 20:19:55
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