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Thursday, June 28, 2012

How Diamond Crystals Grow

A diamond crystal from Crater of Diamonds State Park, Arkansas
Photo by Rob Lavinsky

Diamonds don’t grow overnight because in nature their growth proceeds one atom of carbon at a time with four carbon atoms bound together by chemical bonds.  The natural way that this bonding occurs will normally create a crystal that is an octahedron in shape.  This shape however can be altered by several different parameters imposed by the space allocated to the growth of the crystal in the media in which it is formed.  In the process of growing diamonds are apt to include small crystals of the surrounding material into their crystal shape.  These inclusions are literally windows into the mantle of the earth that are brought to the surface when the diamond is carried to the surface of the earth by a kimberlite or lamproite.

Diamonds are nothing more complicated then a crystal of pure carbon, and are in fact an allotropic form of an element that can have other crystalline forms either amorphous as lampblack or as graphite from which we make pencils.  There is another rarer amorphic form of carbon termed ‘lonsdalite’ that is somewhere between graphite and diamond in physical structure.  Lonsdalite is not a variety of diamond but is instead a different material.  In lonsdalite there is a repeating networking of carbon atoms that all point in the same direction rather then alternating back and forth as in a diamond.  Its crystals instead of forming cubes or tetrahedra as the diamond does are instead hexagonal.  Although it is often found with diamonds it is considered to be an allotropic form of carbon formed as the result of shock from a large meteorite.

From work performed on some of the diamonds from the Ekati diamond mine in the Northwest Territories of Canada it would appear that the growth of diamond crystals begin with a small fragment of graphite upon which the diamond crystals are deposited over time.  A matter of controversy is the actual speed in which diamond crystals are formed.  Experiments in Japan with manmade diamond crystals suggest that it is possible to grow a ten caret crystal in several hundred hours.  This of course is under ideal conditions and in nature would probably take much longer as the diamond crystal would have to scavenge its carbon atoms from the surrounding rocks.  Carbon is a rare element in the earth’s rocks.

Most of the carbon available to create a diamond has been postulated to come from oceanic crust that has been subducted beneath a continental crust as deceased life forms of coral reefs with the necessary temperature to cause their disassociation into the components making up the coral reef.  This would be carbon and the carbonate radical.  The calcium or magnesium from this reaction would be incorporated into the earth’s mantle.  Whatever free carbon remains from this reaction would be available at this temperature and pressure to form diamond crystals.

As the diamond crystal grows it is also scavenging other elements that are incorporated into the crystal as crystal defects.  The most common of these elements are nitrogen and iron.  These elements cause the diamond crystal to become colored with the coloration being dependent upon the amount of these elements.

Harlow, George E., The Nature of Diamonds, American Museum of Natural History, © 1995
Mineralogical Association of Canada, Editor Lee A. Groat, Geology Of Gem Deposits, Short Course Series, Volume 37, Yukon Geological Survey, © 2007, Yellowknife, Northwest Territories
Gem Deposits,
Geology of Gem Deposits, Mineralogical Association of Canada, Editor Lee A. Groat,  Volume 37 © 2007

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