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.

 


 

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