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36
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THE MATRIX OF THE DIAMOND
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and trachyte).1
In both cases the titanic spinellid or ilmenite is surrounded or
penetrated by the younger sphene or perovskite, silica being necessary
for the formation of sphene out of titanic iron, but not for that of
the more basic
perovskite.
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The
other minerals, which are to be found in examining the specimens from
Kimberley, with the exception of the diamond, are less important.
Magnetite, chromite, titaniferous-magnetite, and pieotite, —four isomorphous minerals with difficulty distinguished from one another2—occur
in small grains or crystals scattered through the ground-mass. Chromite
is, perhaps, the most abundant of the four, and is primary, while
magnetite may in part result from the decomposition of the olivine. The
chromite is abundant in minute octahedrons. It is a mineral to be
expected in a magnesian rock, occurring in almost all serpentines.
Pieotite or pleonaste occurs in similar minute grains, the latter of a
dark green colour in transmitted light. Both the chromite and the
spinel-mineral are much smaller than the perovskite crystals.
Another
black mineral in octahedrons occurs in association with rutile.
Ilmenite proper is, of course, rhom-bohedral, and usually is in thin
plates. This mineral may be the same that so often occurs in basalts,
and was called ' trapp-eisenerz ' by Breithaupt. In titaniferous
magnetite
1 Sphene also occurs in claolite-syenite and phonolite.
2 For some remarks on this point see Wadsworth, Lithological Studies, section vii.
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