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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
62
A POPULAR TREATISE ON GEMS.
other, in reference to a particular face of the crystal which forms the plane of union, or the equator of the made. We may also suppose the two crystals originally parallel, and the one turned round the normal of the united faces by 180° (often 90° or 60°), while the other is stationary. Or we may suppose a crystal cut into halves in a particular direc­tion, and one half turned 180° on the other; and hence the name of hemitrope given to them by Hauy. The position of the two individuals in this case corresponds with that of an object and its image in a mirror, whose surface then represents the plane of union.
The manner in which the crystals unite also differs. Some are merely opposed or in simple contact; others are, as it were, grown together, and mutually interpenetrate, occasionally so completely as to appear like one individual. The twin edges and angles in which the two unite are often re-entering; or they may coincide in one plane, when the line of union is either imperceptible, or is only marked by the meeting of two systems of striae, or other diversity in the physical characters of the two faces.
The formation of twin crystals may be again repeated, forming groups of three, four, or more. When the faces of union are parallel to each other, the crystals form rows of indeterminate extent; where they are not parallel, they may return into each other in circles, or form bouquet-like or other groups. Where crystals are merely in juxtaposi­tion, they are sometimes much shortened in the direction of the twin axis; and where many occur in a series with parallel position, are often compressed into very thin plates, frequently not thicker than paper, giving to the surface of the aggregate a peculiar striated aspect.
Only a few twin crystals in the different systems can be noticed, chiefly as examples of this mode of formation. In the tesseral system, forms that unite with parallel axes pro-
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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