Igneous Processes
Diamond Pipes
Diamond deposits are unique in that they are found associated with an unusual type of igneous rock that forms from magmas originating at depths of 75 to 125 miles (120 - 200 km) in the upper mantle. As indicated in Figure 12.4, the pressure and temperature at this depth can cause carbon atoms to arrange themselves into a stable atomic structure (Chapter 3), forming individual crystals of the mineral we call diamond. Closer to the surface, the stable mineral structure for carbon atoms is that of graphite one of the softest substances, whereas diamond is the hardest. Once formed, the diamond crystals can be carried by magma that forces its way toward the surface through fissures. Because the magma is highly pressurized, it decompresses suddenly and violently (Chapter 6) when encountering a weak zone in rocks near the surface. The result is a small, but very explosive volcanic eruption that produces a carrot-shaped crater known as a diamond or kimberlite pipe (named after the town of Kimberly in South Africa). The pipes themselves are filled with magnesium-rich volcanic rocks that generally are quite different from the surrounding rocks.
Diamonds are relatively rare because the formation of Kimberly pipes is geologically somewhat uncommon, plus not all pipes contain diamonds. Interestingly, the volcanic debris lying on the surface. Diamonds were later discovered by gold prospectors in stream gravels that had been carried off the landscape by running water. Eventually people recognized that diamonds were associated with the circular volcanic deposits. This association led to the discovery of new deposits and the mining of diamond-bearing rocks from within the kimberlite pipes themselves. Note that kimberlite pipes tend to occur in swarms or clusters, which is why diamond mines are usually found within small geographic areas, often called mining districts.