Each of the following is mentioned in the passage as a difference between G10 garnet and other versions of garnet EXCEPT
A.
level of oxidation
B.
commonness of occurrence
C.
chemical signature
D.
place of formation
E.
appearance in conjunction with diamonds
部分文章
In the 1970’s the process of locating profitable pipes was refined by focusing on the subtle differences between the chemical
signatures of indicator minerals found in diamond-rich pipes as opposed to those found in barren pipes.
For example, G10 garnets, a type of garnet typically found in diamond-rich pipes, are lower in calcium and higher in chrome than garnets from barren pipes.
Geochemists John Gurney showed that garnets with this composition 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 into carbon dioxide. 作者: honeybee316 时间: 2012-10-16 21:11
我看这段话里只提到了选项C -- lower in calcium and higher in chrome，应该还有其他的段落吧？作者: baochangzx 时间: 2012-10-17 06:27
这是完整的段落，非常感谢~
Diamonds are almost impossible to detect directly because they are so rare: very rich kimberlite pipes, the routes through which diamonds rise, may contain only three carats of diamonds per ton of kimberlite.
Kimberlite begins as magma in Earth’s mantle (the layer between the crust and the core).
As the magma smashes through layers of rock, it rips out debris, creating a mix of liquid and solid material. Some of the solid material it brings up may come from a so-called diamond-stability field, where conditions of pressure and temperature are conducive to the formation of diamonds.
If diamonds are to survive, though, they must shoot toward Earth’s surface quickly.
Otherwise, they revert to graphite or burn. Explorers seeking diamonds look for specks of “indicator minerals” peculiar to the mantle but carried up in greater quantities than diamonds and eroded out 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 to mine.
In the 1970’s the process of locating profitable pipes was refined by focusing on the subtle differences between the chemical
signatures of indicator minerals found in diamond-rich pipes as opposed to those found in barren pipes.
For example, G10 garnets, a type of garnet typically found in diamond-rich pipes, are lower in calcium and higher in chrome than garnets from barren pipes.
Geochemists John Gurney showed that garnets with this composition 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 into carbon dioxide. 作者: baochangzx 时间: 2012-10-17 21:04
看了一下后发现线索还是在第二段中。
Geochemists John Gurney showed that garnets with this composition were formed only in the diamond-stability field；more commonly found versions came from elsewhere in the mantle.这里的garnets with this composition指G10，而more commonly found version即other versions of garnets。这句话可以推出D)选项。B)也可以出这句话得出，注意其中的from elsewhere in the mantle可以让人感觉到其出现的普遍性。
E)是从这个句子推导出来的G10 garnets, a type of garnet typically found in diamond-rich pipes。说明了他是通常和diamond混一起的。
oxidization是相对于ilmenite中的iron而言而不是G10 garnet，所以需要选A)。 when the iron in ilmenite was highly oxidized, its source pipe rarely contained any diamonds.作者: shanexwang 时间: 2012-10-18 06:33
