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Ch. 1: Form of Minerals

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64                     A POPULAR TREATISE ON GEMS.
represented in fig. 73. In fig. 74 of zinc-blende, two rhom­bic dodecahedrons are united by a face of the octahedron. In the Tetragonal system, twin crystals with parallel axes
rarely occur, but are seen in chalcopyrite, and one or two other minerals. Where the axes are inclined the plane of union is very often one of the faces of the pyramid Pco , or one of those faces that would regularly replace the polar edges of the fundamental form P. The crystals of tin ore obey this law, as seen in fig. 75, where the individuals are pyramidal, and in the knee-shaped crystal (fig. 76), where
they are more prismatic. Hausmanite appeal's like fig. 77, in which the fundamental pyramid P prevails, on whose polar edges other crystals are often very symmetrically repeated.
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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