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.