Form of Minerals Feuchtwanger: Treatise on Gems Page 69



	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