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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
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A POPULAR TREATISE ON GEMS.
(with a rhombic strias) is parallel to the macropinacoid {q) of the other.
In the monoclinohedric system the most common macles are those in which the principal axes and the chief sections of the two crystals are parallel to each other, and conse­quently the principal axis is also the twin axis. Usually the two individuals are united by a face parallel to the or-thodiagonal chief section, as in figure 89 of gypsum, where two crystals of the combination shown
in fig. 59, unite so regularly that the faces of the pinacoids (P and P') form only one plane. In a similar maimer the augite crystals of the combination P, represented singly in fig. 60, are in fig. 90 united in a
made so very symmetrical and regular that the line of junction cannot be observed on the face of the clinopinacoid. The two hemipyramids in the gypsum
crystal above) form on one side a re-entering, on the other a salient angle. Hornblende, wolfram, and other minerals exhibit a similar appearance. In other cases the individuals partially penetrate each other, being, as it were, crushed together in the direction of the orthodiagonal. This mode of union is not uncommon in gypsum, and very frequent in orthoclase felspar. Two crystals of the latter, of the com-
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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