The
Mysteries of Eastern Australian Diamonds
Dr Julian D. Hollis B.Sc
(Hons) Ph.D (London)
Are you a
fellow diamond enthusiast? If so I would like to share some of my
aesthetic pleasure derived from this beautiful gemstone. My approach is
that of a geo-scientist working as a consultant to the gemstone industry,
especially the Melbourne Diamond Exchange.
My in-line to diamonds is via their crystallography
– beautiful natural shapes reflecting their formation, provenance and
tortuous history in volcanic systems.
Like snowflakes, no two crystals are ever the same.
– Unique expressions of natural artistry.
Crystallography of Diamonds from Eastern
Australia
Classification
|
Primary
Forms |
Resorbed
Forms |
Etching |
Twins |
|
C:
Cube
D:
Dodecahedron
F:
Tetrahedron
O:
Octahedron
/
Combination |
HC:
Tetra hexahedron
HD:
Pseudo dido decahedron
HF:
Hex tetrahedron
HO:
Hex octahedron
TD:
Pseudo dodecahedron
TO:
Trisoctahedron |
-c
cubic
-hc
tetrahexahedral
-o
octahedral
-to
trisoctahedral
|
2 x
5 x |
|
| In Eastern Australia we
have a great geological mystery, shared by Alaska, California,
Kamchatka, Burma, Thailand and the Urals: Theoretically diamonds
should not be concentrated in the “blind placers” of such
places. Most diamonds apparently come from stable ancient crust
areas – cratons, beneath which the rocks are relatively cool,
within the stability field for diamond. Being meta-stable, diamond
readily converts to graphite or just burns to carbon dioxide
unless stringent protection can be afforded. High temperatures
cause destruction of diamond if upward transport is too slow,
releasing pressure without appropriate cooling rates.
Eastern Australia presents a
puzzling situation where certain local areas yield abundant
diamonds, at least 500,000 carats from the Copeton and Bingara
Fields of Northern New South Wales alone. Prior to production from
Argyle , Western Australia this was the only commercial production
in Australia. But ; one big problem: The NSW diamonds occur over
1000 km from the nearest craton, straddling a geothermally hot
“mobile belt” left over from a major mountain – building
episode (orogeny) from some 400 to 200 million years ago. This
does not appear to be a suitable place to find a primary
diamond-bearing volcanic system! But;
how does one explain local concentrations and peculiarities of
form characterising each find?
Long distance alluvial travel
or ice carting dilute and scatter diamonds to uneconomic grades.
|
C: Cubes
HC-o |
C/D-o |
TD-o
TWIN:
2X TD-c |
| F: Tetrahedra |
D: Dodecahedra |
|
HF-hc |
No primary forms
confirmed
C/D, C/O, C/D/O
Combinations not recorded |
|
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