November 24th, 2008
Geology MW 7:45
Diamonds are a Girl’s Best Friend
A diamond is considered to be one of the most precious and rare stones on earth. These stones “are rare for three reasons: they form deep within the Earth in very localized places beneath continents, an extremely unusual kind of volcanic activity is needed to bring [diamonds] to the surface, and only about 20 per cent of them are gem quality” (Fleet, Hart, Wall 23). Not all diamonds are good enough to sell for jewelry purposes but are good for different industrial uses. Many extensive geological processes are involved in the formation of diamonds as well as long periods of time needed. “Diamonds which are mined formed about three billion years ago” (Fleet, Hart, Wall 23).
The processes that occur in diamond formation are not completely known but different geologic evidence indicates certain processes that have taken place. Most areas that contain a significant amount of diamonds are older areas within the earth’s crust containing magmatic basaltic rocks.
Diamonds are mostly found in kimberlite pipes. A kimberlite forms in the mantle within the earth along with a substantial amount of volatiles. A “free carbon system” is also an essential need in the formation of diamonds. Between the extreme temperature and pressure along with the volatiles (possibly water and carbon dioxide), the magma pushes upward and forms a pipe that is compared in shape to a carrot. The actual pipe forms in that process. This is a rare type of volcanic explosion that no person has ever seen, but can be seen in geologic history. Also a kimberlite pipe is a type of diatreme, which is another type of pipe filled with breccias from a gaseous explosion. A kimberlite rock formed in the mantle of the earth and can reach temperatures up to 900 degrees Celsius (Pokhilenko 28-29). For diamonds to come up to the surface via kimberlite pipes, they have to be “tectonically activated” and the pipe will eventually connect with the “earth’s surface (Pokhilenko 31).” “The ocean floor has to slide into the mantle [and] the basaltic rock becomes eclogite, and the organic carbon may become diamond” (About Diamonds 2).The magma from the volcano “originates in the earth at depths of more than 150 km,” likely at “the base of, or just below the thickened plates of the cratons, far deeper than other volcanic magmas.” “They [the magma] are said to find their way through fractures in overlying rock and scavenge pieces of debris from these rocks as they ascend.” In the mess is diamonds. The process of the upward moving magma is extremely slow and for diamonds to form, “acceleration in essential in ensuring that the diamonds neither alter to graphite as the pressure and temperature decrease, nor are oxidized to carbon dioxide” (Fleet, Hart, Wall 23). Diamonds are carried to the surface in “large pieces of mantle rocks called xenoliths” (Diamond 1). Over time when the diamonds have reached the surface, they can erode and end up in places such as streams or end up being carried away in the ocean.
When diamonds are being formed within the earth, a C-center is first formed which is when the “diamond captures nitrogen in the form of separate atomic centers. Then after a process like diffusion, the centers come together and form an A-center. Next, the A-centers come together and form a B-center. A diamond can end up with nitrogen inclusions from formation but those can go away if there is high enough pressure and temperature (Pokhilenko 29).
An area that contains diamonds can be detected by “magnetic anomaly” because some of the areas are surrounded by magnetic materials (Pokhilenko 28). This test is not a definitive test, rather a possible indicator of a diamond-bearing location. One article I read about talks about how a huge kimberlite pipe in Canada was almost missed because it did not test positive for the magnetic materials. Others tests would be needed to determine if...
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