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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.                                69
binationas in fig. Gl above, are
often pushed sidewise into each other, as shown in fig. 91.
In the triclinohedric system, some twin formations are ot great importance as a means of distinguishing the triclino­hedric from the monoclinohedric species of felspar. In one variety the twin axis is the normal to the brachydiagonal chief section. But in the triclinohedric felspars this sec­tion is not, as it is in the monoclinohedric species, perpen­dicular to the basis, and consequently the two bases form on one side a re-entering, on the other a salient angle; whereas in the monoclinohedric felspars (where the brachy­diagonal chief section corresponds to the clinodiagonal), no twin crystals can be produced in conformity to this law, and the two bases fall in one plane. The albite and oligoclaso very often exhibit such twins, as in figure 92, where the
very obtuse angles formed by the faces of OP, or P and P' (as well as those of, are a very charac-
teristic appearance, marking out this mineral at once as a triclinohedric species. Usually the twin formation is re­peated, three or more crystals being combined, when those in the centre are reduced to mere plates. When very nu­merous, the surfaces P and x are covered with fine stria?, often only perceptible with a microscope. A second law
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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