Diamonds are almost impossible to detect directly because they are so rare: very rich kimberlitepipes, 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 rea- 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 available oxygen. When iron became highly oxidized, so did diamonds; that is, they vaporized into carbon dioxide.
The primary purpose of the passage is to
*内容概述： 文章开头说女性的草根运动时期和他们对新文明意识的看法的核心来自于美国在大推进时期的社会改革。接着进一步解释在这一时期中产白人女性改革者取得的成就。接下来笔锋一转，在童工问 题上不同阶级女性看法不同。改革者认为必须剔除，但工人阶级则认为童工法会让家庭里干活的人变少，生活难以为继。最后是作者的评价，承认改革者要求不适用童工是正确的，但他们没有考 虑到工人阶级的家庭经济状况 *文章类型：人文历史 *文章套路：对比解释说明 --------------------------------------------------------------- *题目类型：主旨题 *选项分析：主旨题去看全文内容概述。本文的目的就是文章的结尾处说的中产阶级改革者没能够理解公认阶级的经济状况。因此正确答案是D
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