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At various points along the
Eastern Australian mobile belt there are strange relics of
sub-volcanic structures, some directly associated with diamonds.
These range from narrow fissures to large basin like features,
filled with ground up:”Tuff sites”. Tuff sites are
gas-dissociated rocks that have been churned and intruded.
Associated diamonds show minimal abrasion and strong rounding,
also a high degree of polishing due to surface melting. They are
unlikely to have travelled far. But, back to the crystals:
An Australian company
exploring for diamonds recently recovered some 8500 diamonds from
a test program in the area. These included an unusually fine suite
of crystals.
Curiously the larger diamonds seldom show clear
crystal form. It is distinctly unusual this way around, even more
so as there are no sub-1mm micro diamonds.
Crystal types are also
abnormal; being dominated by highly rounded forms resulting from
magmatic resorption. These are frequently sculptured to oddly
shaped 12 sided Dodecahedra or 24 sided Dido decahedral forms.
The
normal Octahedra are extremely rare, in sharp contrast to their
dominance in Kimberlitic systems. However these too are present
and show the usual progressive resorption from crystal face edges
(coigns).
Many crystals are twinned, with planes forming ultra
–hard zones (naatz). Other naatz reflect planes of stress formed
under intense shearing in upper mantle, over 120 km (75miles)
down.
Diamond crystal forms appear
to be dictated by depth (pressure) and temperature conditions
during formation. Those from shallow levels are mostly cube forms
which are extremely rare in Eastern Australia.
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