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THE MATRIX OF THE DIAMOND 33
in
the perovskites of other localities. The striation parallel to the
cubical faces, extinguishing when the section is at 45° to the
polarisers, occurs also on perovskite from Zermatt, from the Urals
(where also all the crystals are penetration-twins), from the Tyrol,
from Arkansas, from Wiesenthal (Erzgebirge), &c. The figures of
perovskite grains out of the nepheline-basalt of the last locality, as
given by Saner,1 are very similar to those given above from the Kimberley mineral.
Perovskite
also occurs in the Kimberley peridotite in aggregates of crystals and
in irregular grains. Very frequently these crystals or grains enclose
one or more opaque black octahedral crystals of a titaniferous
magnetite (or other spinellid). Often an octahedron or cube of
perovskite will have a black grain in the very centre, and when an
aggregate of such crystals occurs, each of its components may have a
black grain in its centre.
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This
is so common an appearance that one is led to suspect that the
perovskite is a secondary mineral, made out of the older titanic iron
through some reaction with the basic magma. While, generally, the
titanic iron is in the form of small grains in the perovskite, as shown
in the above figures, sometimes the perovskite forms a narrow fringe on
one side of a larger mass of titanic iron, or makes a shell around it.
Sometimes also a crack in a mass of titanic iron is filled with
perovskite, as if by a secondary mineral.
That
titanic iron is older than the perovskite is proved by the fact that
while titanic iron occurs as an enclosure in the olivine, perovskite
never does so.
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