Physics and Chemistry of Impact Diamonds: Examples
from Popigai
Andrew
Phelps (Material Physics Lab, University of Dayton, Dayton, Ohio, USA)
2009 Midwest Chapter of the Friends of Mineralogy
Symposium and Field Conference (Geology Department of Miami University, Oxford,
Ohio, USA)
5 September 2009
There
are 2 kinds of diamonds.
Interstellar
diamonds are the oldest objects we can have. They have isotopic
signatures different from Earth solar system norms. They can’t be dated
directly.
Diamond origins:
1)
mantle origin
2)
metamorphic origin in exhumed terranes (Ex: Dabei Shan) - organic carbon
metamorphosed into diamond. These have been found in Norway and China.
3)
plasma vapor formation in interstellar medium (presolar)
4)
plasma vapor formation from meteorite impact (Ex: Popigai)
5)
high-pressure impact formation in space - these are in meteorites
themselves (Ex: Canyon Diablo). They are not necessarily formed
in-situ, but they got in there.
Sloan
Ranch, Colorado - can find high-pressure diamonds there. Blackstone at
Sloan was quarried out. It was found to sometimes chew up rock
saws. So, diamonds are found in the rocks there.
Crater of Diamonds
in Arkansas is a lamprophyre,
not a kimberlite.
Kimberley,
South Africa - the kimberlite pipe there had an alkaline lake/seep above it -
it killed wildlife.
The
Crater of Diamonds site was originally a swamp.
Can
synthesize diamonds using a variety of techniques. Can even synthesize
them with an ordinary welding torch.
Lonsdaleite
is a hexagonal
“diamond” - a carbon polymorph.
Diamond types
1)
natural cubic diamond
- often with graphite inclusions. Trigons are
structures along the 111 plane of a diamond. When diamond is heated, it
will break along the 111 plane, and graphite will form on that plane.
Graphite is never found cross-cutting the 111 plane. The cubic character
comes from trigonal structure when viewed at a certain angle.
2) hexagonal diamond
(lonsdaleite) - discovered in some meteorites (Canyon Diablo [scroll down]
was destroying rock saws) and craters after its existence was predicted by
Ergun & Alexander (1962). The Allende Meteorite has diamonds, but you
can’t readily tell that because carbonaceous
chondrites fall apart easily when cut with rock saws.
Polytypes
- a special type of 1-dimensional polymorphism. Crystal structure is
built from stacking identical layers. In repeating sequences. Unit
cell dimension changes in only one direction.
Diamond
polytypes - there are other kinds of carbon polymorphs. 6H diamonds were
identified based on C.E. Holcomb’s 1973 report. There are also 4H and 8H
diamonds.
You
can have diamonds that don’t x-ray as cubic.
Moissanite (SiC) - all of
it is manufactured in North Carolina.
Arkansas
diamonds are not harder than other diamonds.
Like
SiC, diamond occurs in many polytypes.
Space
diamonds - diamond formation in space is not well understood. Some
diamonds in meteorites and chondritic porous aggregate micrometeorites are
believed to have survived since before solar system formation. Diamond
appears to be a very common component of interstellar dust.
Allamandola
et al. (1993) found a 2880/cm band in 4 protostellar clouds, demonstrating
diamonds appear to be abundant and ubiquitous in interstellar space.
Diamond
may be the most common solid material in the universe. Diamond forms
around carbon stars.
Diamonds
hang out a long time.
Extrasolar
diamonds are enriched in xenon-HL - both the heaviest & lightest
isotopes. Extrasolar diamond grains have trapped in them anomalous exotic
noble gas isotope mixtures.
Primitive
meteorites have up to 400 ppm of a very fine-grained type of carbon (C∂)
- comes from outside the solar system. Primitive meteorites have 2
classes of diamond found in them, including C∂. C∂ is
diamond, but is a class down.
Diamonds
in meteorites have >1 source.
Diamonds
on the ground that are associated with craters may or may not be meteoritic
material.
Looked
at the Popigai Impact Crater in Siberia [photos & info. -
scroll down]. Impact craters tend to be large round swamps, like
kimberlites. Popigai is a big, circular muckhole.
The
Russians found a 60 mile-across mantle uplift at Popigai.
Can
still find a suevite layer at Popigai. When dissolved down, get
microdiamonds. Can buy Popigai suevite.
Popigai
hexagonal diamonds have internal slip planes (crystallographically
controlled). These diamonds formed and then were deformed.
Popigai
hexagonal diamonds have graphite on internal faces/planes. Some are well
formed, crystal-wise. Others are beaten up and fractured. Popigai
diamonds formed instantaneously.
Popigai
diamond abundance is related to the abundance of coal beds in the Popigai
impact target area.
Vishnevsky
et al. (1999) - Impact Diamonds, Their Features, Origin and Significance.
Lonsdaleite
= 2H diamond (these are interchangeable labels)
Definition
of diamond - SP3 bonded carbon or cubic diamond only.
