26 THE MATRIX OF THE DIAMOND
alternate striae is about 10°, showing an extinction for single lamellae of 5°.
The
mica seldom occurs in well bounded crystals, being more usually found
in irregular plates. Sometimes the crystals are corroded, as if from a
re-solution of the magma. Sometimes also a narrow rim of 'opacite'
surrounds the mica, this being particularly observable in such crystals
as have been greatly corroded and rounded. This black rim (the 'opacit
rand') is never thick, and is only sometimes present. Sometimes the
edges of the mica crystals are curved, this being the result of motion
in the rock. In certain cases the mica is found to contain very small
short thick rods of a highly refracting dark red substance, probably
rutile. These rutile needles lie parallel to the base, and are probably
a result of the partial decomposition of the mica. They indicate that
the mica contained a small percentage of titanium. The production of
rutile as a secondary mineral has been frequently observed in
magnesia-mica.1
Another
mode in which the mica occurs is around the garnets in the rock.
Sometimes a large red pyrope will be entirely surrounded by a narrow
zone of brown mica.
A
third method of occurrence of the mica is in the form of small scales
in enstatite, here apparently due to the alteration or corrosion of
that mineral. This was noticed in only a few instances, and especially
in those in which the enstatite was enclostd in olivine. In one
instance, small scales of magnesia mica alone were enclosed in olivine,
as though the original enstatite had been entirely replaced by the mica.
A fourth association in which we find magnesia-mica is as a product of the apparent fusion or metamorphism of
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