Explorers seeking diamonds look for specks of

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Explorers seeking diamonds look for specks of “indicator minerals” peculiar to the mantle but carried up (25) in greater quantities than diamonds and eroded out of kimberlite pipes into the surrounding land. The stan- 24
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dard ones are garnets, chromites, and ilmenites. One can spend years (30) 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 (35) are usually too sparse to mine. In the 1970’s the process of locating profitable pipes was refined by focusing on the subtle differ- ences between the chemical (40) 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 (45) diamond-rich pipes, are lower in calcium and higher in chrome than garnets from barren pipes. Geo- chemists John Gurney showed that garnets with this composition were (50) formed only in the diamond-stability field; more commonly found ver- sions came from elsewhere in the mantle. Gurney also found that though ilmenites did not form in the (55) 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 rea- (60) soned that iron took on more or less oxygen in response to conditions in the kimberlitic magma itself—mainly in response to heat and the avail- able oxygen. When iron became (65) highly oxidized, so did diamonds; that is, they vaporized into carbon dioxide. -------------------------------------------------------------------------------- Q23: The primary purpose of the passage is to A. discuss an objection to Gurney’s theories about the uses of indicator minerals B. explore the formation of diamonds and the reasons for their scarcity 25
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C. analyze the importance of kimberlite pipes in the formation of diamonds D. define the characteristics of indicator minerals under differing conditions E. explain a method of determining whether kimberlite pipes are likely to contain diamonds Answer: -------------------------------------------------------------------------------- Q24: Each of the following is mentioned in the passage as a difference between G10 garnet and other versions of garnet EXCEPT -------------------------------------------------------------------------------- Q25: The passage suggests that the presence of G10 garnet in a kimberlite pipe indicates that --------------------------------------------------------------------------------
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