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.
Classic localities:
1)
India
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.