Diamonds - Their Geology, Geography, and History
Karen Rice (gemologist for Suna Brothers, Inc., New York, New York, USA)
2004 Central Ohio Mineral, Fossil, Gem & Jewelry Show (Columbus, Ohio, USA)
27 March 2004
Diamonds are first noticed by man in ~800 B.C.
Diamonds form ~150-200 km below the surface (in the diamond stability field) - at those depths, carbon crystallizes out as diamond.
E-type diamonds - form in eclogite.
P-type diamonds - form in peridotite.
Diamond inclusions are used to tell P-type from E-type diamonds. P-type diamonds are more common (3:1 ratio of P-type to E-type diamonds).
Diamond formation doesn’t occur in pipes (such as kimberlite diatremes). There are 3 other types of diamond-bearing pipes besides kimberlites (including lamproites).
Crater facies of a kimberlite - 50 meters high; only one kimberlite crater is known in the world - they weather quickly, so don’t survive long.
Diatreme facies of a kimberlite - the pipe itself. The pipe is typically 2500 meters long (from crater facies to the root zone, if crater facies is still preserved).
Root zone - 2-3 km below the surface, if the crater facies is preserved at the surface.
Diamonds will only be present in kimberlite pipes if the magma is rising at a certain velocity. If the magma is going too slowly, the diamonds revert to graphite. 20-30 km per hour is the necessary velocity of rising magma to maintain diamonds. So, it would take 5-15 hours for magma to rise during a kimberlite pipe formation event.
Kimberlite pipes have a steep cone shape - the cone drops down at an ~80° angle.
DeBeers Diatreme - has 3 feeders in the root zone.
Diamondiferous kimberlites range in age from 1.6 b.y. to 50 m.y.
The diamonds are older than the pipes. How much older? Can’t date the diamonds directly. However, you can date the inclusions in a diamond. Diamonds are known to range in age from 3.3 by to 990 m.y.
Diamond pipes only occur on cratonic regions of the world (old/stable landmasses). Example: Kapvaal Craton of South Africa - Africa’s oldest craton, at 3.5 b.y.
Diamond pipes occur only in the oldest parts of the craton (Archean bedrock). Can occasionally have diamondiferous pipes intruding younger rocks, though.
110-120 million carats worth of diamonds are mined every year. Most of these are industrial-grade. Gem-quality diamonds are rare.
One rarely sees diamond crystals for sale, though.
Diamonds crystallize in the cubic system. The common crystal shape is an octahedron. Gem-quality cubic crystals are rare Many gem-quality diamonds have no mineral crystal shape.
Before cutting, shapeless samples have to be oriented with respect to crystallographic axes (orientation is necessary for cutting purposes - cleavage planes could interfere with cutting + the octahedral face of a diamond can’t be cut - too hard for any known cutter/polisher).
Bort - industrial-quality aggregate diamonds - it is crushed up & used in industry.
Can get crystal twins.
Terraces/steps on crystal faces - indicate diamonds grow in layers. Layers can preferentially get deposited on crystal faces over edges or edges over faces.
Trigons (triangular pits) only appear on octahedral faces (the hardest face).
If a diamond travels more slowly up a pipe, the magma attacks the diamond and it acquires a ball-like shape.
Green spots or green skin - surficial damage due to radiation in the rock.
All diamonds were derived from alluvial mining until 1930, when the first mining of pipes started.
2) Brazil - diamonds were found in 1725 in southeastern Brazil. All alluvial mining in Brazil, even though diamondiferous pies were found there in 1965.
3) South Africa - the main diamond rush was in the mid- to late-1800s in southern Africa. Example: inland South Africa, near Barkly - landscape has little vegetation. But, there are some depressions in the land where water accumulates and some vegetation grows. These depressions are called “pans”. The Star of Africa diamond started the southern Africa diamond rush in the 1860s. This diamond rush was along the banks of the Orange River, just northwest of Hopetown. At first, just looked for diamonds in & along the Orange River. Then, started looking for pans - that’s where the diamonds were coming from. The pans were the weathered tops of kimberlite pipes. Kimberlite is a soft rock, so it weathers quickly, and the top of the pipe is lower than the surrounding land surface. Kimberly region - has the richest cluster of diamond pipes in the world. 1871 - found about a dozen pipes in the Kimberly area. Weathered kimberlite = “yellow ground” (soft & easy to mine). Unweathered kimberlite - “blueground” (harder rock & harder to mine). Mined blueground rock was left outside to weather into yellow ground. There was little access to water in these mining areas, so miners engaged in “dry digging” and “dry mining”. Weathered yellow ground was dry-sifted for diamonds. The Big Hole at Kimberly - closed in 1940. Several other kimberlite pipes occur in southern Africa, in places like Botswana, Tanzania, Lesotho. Premier Mine (now called Cullinan Mine) - discovered in 1903. It is a cluster of 21 pipes. The largest rough diamond in the world derived from here - 3106 carats, but that rough sample was only part of a larger diamond (had been naturally cleaved) - the rest of that crystal was never found. This 3106 carat rough diamond was cut into 9 major stones. The Premier Mine has produced 300 stones that were all over 100 carats. The base of the pipe at the Premier Mine is fed by multiple feeder dikes (some as small as 2 feet across); there were 15 pulses of kimberlite through these basal feeder dikes. The diamonds from the Premier Mine are almost the same age as the kimberlite pipe itself. Kimberlite has an average weathering rate of 1 meter per 30,000 years. Much kimberlite material has weathered down the Orange River drainage basin - it has been draining the region for 100 m.y. So, there are diamond-bearing gravels along the coast. The South African-Namibian coastline has diamond-bearing gravels that are mined. Since the 1866 discovery of diamonds in South Africa, 20 other countries have started producing diamonds.
4) Canada - the biggest diamond rush since South Africa has been in Canada. In 1920, diamonds were discovered in Ontario. Lac de Gras in the Northwest Territories has been an area put into diamond production - a big push for diamond exploration & exploitation there started in 1991. There are 100,000 acres under claim there.
5) Russia - diamonds were first found in the Ural Mountains in 1829 (alluvial). The first diamond discovery in pipes was in 1954 in Siberia. Arkhangelsk - an active exploration area in northwestern Russia.
6) China - diamonds were found in the Shandong region in the early 20th century. Guizhou has a diamondiferous pipe. Hunan has diamondiferous pipe & alluvial diamond exploitation.
Antarctica - the East Antarctic Craton has several identified regions of Archean age (archons). But, Antarctica is off-limits to any mining by international treaty, and will probably stay that way.