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Matrix of the Diamond

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30                 THE MATRIX OF THE DIAMOND
high as rutile (over 2) and a power of double refraction ahout as low as zoisite (y — a = -005). The low douhle refraction, together with the opacity often caused hy in­clusions of ilmenite or chromite, might in some cases cause it to he mistaken for a dark spinel or other isotropic mineral. In no case, however, is the perovskite isotropic. Extinction always occurs parallel to the diagonal of the cube. Pleochroism is extremely faint. The form and optical properties of the perovskite show that in all cases it belongs to the rhombic system, and is in the form of twins, or of aggregates of twins. The commonest form is that of interpenetrative twins, so crossed as to form a cube. The following are common forms as seen in ordinary light: —
A cleavage or parting occurs parallel to the cubic faces. Re-entrant angles usually divide the elements of the twins, but these are still better indicated by the optical characters of the mineral. The extinction is always parallel to the diagonal of the cube, as shown by the dotted lines in figs. 21, 22, and 23. The apparent cube appears to be composed of interpenetrating rhombic individuals, whose
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Lewiss. Genesis and Matrix of The Diamond.
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