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THE MATRIX OF THE DIAMOND
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peridotites,
while the idiomorphic character of the crystals, and the evidence of
the action of a corrosive magma upon them, class the peridotite among
the volcanic rather than the plutonic series.
Pyroxenic Minerals.—Three
clear green minerals, almost identical macroscopically, occur in the
peridotite of Kimber-ley, and are found loose in the decomposed ' blue
ground.' These are smaragdite, bronzite, and chrome diopside or
dial-lage. These minerals are so hard and clear and free from
inclusions that they have been cut as gems. They all have a light grass
green colour, and are unattacked by acid, and all sink in a Thoulet's
solution, having a specific gravity of 3. They can be distinguished
from each other by specific gravity, by blowpipe tests, and especially
by optical means. All are coloured by chrome oxide, as is shown by
blowpipe tests.
Smaragdite.—This
has the hornblende cleavage, and an extinction of about 15°. It has a
fine green colour, and in thin sections it is pleochroic in shades of
green. The mineral has been analysed by Maskelyne and Flight, who found
the composition of loose fragments in the blue ground of Du Toits Pan
to be as follows :
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Bronzite and Bastite.—Of the three related green minerals in the peridotite, bronzite is the most abundant.1 It
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