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Matrix of the Diamond

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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 :—
It seems, therefore, that a series of rhombic prisms are twinned according to two laws at the same time, the apparent cubo-octahedrons being in reality compound
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