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Ch. 1: Form of Minerals

Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals Text size:minus plus Restore normal size   Mail page  Print this page
FORM OF MINERALS.
65
a central individual appearing like the support of all the others. Almost identical forms occur in chalcopyrite.
In the Hexagonal system, twin crystals with parallel axes are common, as in calc-spar, chabasite, haematite, and other rhombohedric minerals. In calc-spar they often form very regular crystals, the two individuals uniting by a plane parallel to the base, so as to appear like a single crystal, as in fig. 78, where each end shows the forms but
in a complementary position ; or in fig. 79 of two scaleno-hedrons B' from Derbyshire. The rhombohedric crystals of chabasite often appear intersecting each other, like those of fluor spar in fig. 73. The purer varieties of quartz or
rock crystal, in consequence of the tetartohedric character of its crystallization, often exhibit twins. In these the pyramid P separates into two rhombohedrons P and z, which, though geometrically similar, are yet physically distinct. In fig. 80 the two individuals are only grown to­gether, but more commonly they penetrate each other in an irregular manner, forming apparently a single crystal. Twins with oblique axes are also common, the plane of union being usually one face of the rhombohedron. Thus in calc-spar two rhombohedrons are often joined by a face
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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