12 THE MATRIX OF THE DIAMOND
Serpentine, calcite, zeolites, chalcedony, and talc, as decomposition products. Also an undetermined mineral, probably cyanite, and finally diamond (scarce).
Olivine.—Olivine
forms the most abundant constituent in the rock, occurring in
porphyritic crystals or rounded grains, which may attain considerable
size. These crystals are sometimes over a centimetre in diameter,
though usually of smaller dimensions, and are sprinkled plentifully in
a serpentinous base. The dark-green colour is so nearly that of the
whole rock that the olivine is not so conspicuous as the mica,
although, as seen in thin sections, it is much more abundant. Many of
the crystals are comparatively fresh, with the hardness and lustre of
unaltered olivine. Others are partly changed into serpentine, while
others, again, are entirely replaced by serpentine.
Except
an occasional grain of a spinellid or, more rarely, enstatite, the
fresh olivine is pure and free from solid enclosures. Occasionally
bubble-like inclusions of gas or glass occur, in the form of minute
elongated rounded cavities. These are often aggregated together in
plantÂlike growths, such as are not uncommon in the olivine of a
porphyritic eruptive rock. They also occur in strings traversing the
crystal. With a high power these inclusions are seen to be of a brown
colour resembling glass (negative crystals ?). The interesting changes
which Prof. Judd ' has shown to occur in the olivine of deep-seated
rocks, whereby secondary enclosures are produced, through the process
which he has called ' schillerisation,' are entirely absent here.
An
unusual feature in the olivine of the Kimberley rock is its perfect
cleavage. This cleavage is so perfect that in ordinary light the
mineral would be at once designated as bronzite or diallage. In
polarised light the high double refraction distinguishes it from
enstatite, and the parallel extinction from diallage or diopside. This
cleavage is
