THE MATRIX OF THE DIAMOND 31
basal
pinacoids form the octahedral replacements of the cube. Very frequently
four individuals are seen grouped to form a single square, dark lines
dividing the deep yellow crystal. The entire cube would then be
composed of six individuals.1
A
second kind of twinning is also common in this perovskite, producing
the appearance with crossing nicols of a series of parallel alternately
coloured bands, like the polysynthetic twinning in plagioclase
felspars. The bands are again often crossed at right angles, as in
microcline. These lamellae are parallel to the cubic faces of the
compound twin. They are alternately pale greenish-blue and pale
greenish-yellow, and are all simultaneously extinguished when they are
turned at an angle of 45° to the nicols. The following are among the
forms which are seen by the use of this apparatus :—
