Diamonds are almost impossible to detect directly because they are so rare: very rich kimberlite pipes, the routes through which...
GMAT Reading Comprehension : (RC) Questions
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.
The primary purpose of the passage is to
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| 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. | What it says: Finding diamonds is extremely hard because there are so few of them. Even in the best locations (kimberlite pipes), there are only tiny amounts. What it does: Introduces the central problem - diamond detection difficulty Source/Type: Factual statement with concrete example Connection to Previous Sentences: This is the opening - establishes the foundation of our analysis Visualization: Imagine a massive dump truck carrying 2,000 pounds of rock material, but hidden somewhere in that enormous pile are only 3 tiny diamonds the size of peas Reading Strategy Insight: This sentence immediately tells us what the entire passage will be about - the challenge of finding diamonds. Start with this clear framework in mind. What We Know So Far: Diamonds are rare and hard to find; they come through kimberlite pipes What We Don't Know Yet: How diamonds form, why they're in kimberlite pipes, how people actually search for them |
Why It's Wrong:
- The passage presents Gurney's theories as accepted scientific findings, not controversial ideas facing objections
- No alternative viewpoints or criticisms of Gurney's work are mentioned
- The tone is explanatory and supportive of the scientific advances described
Why It's Right:
- The passage consistently discusses kimberlite pipes as the key to diamond location and evaluation
- The structure moves from explaining what kimberlite pipes are to how scientists determine their diamond potential
- The methodological advances described are all about distinguishing diamond-rich pipes from barren pipes
Why It's Wrong:
- Defining characteristics is a supporting detail, not the main purpose
- The passage mentions indicator mineral characteristics to support the larger point about pipe evaluation
- The focus is on how these characteristics help distinguish promising from unpromising locations
Why It's Wrong:
- This is too narrow - it focuses only on one part of the methodology rather than the complete evaluation process
- The passage covers both positive indicators (G10 garnets) and negative indicators (oxidized ilmenites)
- The method encompasses the entire kimberlite pipe evaluation process, not just indicator mineral detection