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Ch. 1: Form of Minerals

Ch. 1: Form of Minerals Page of 515 Ch. 2: Minerals: Physical Properties Text size:minus plus Restore normal size   Mail page  Print this page
74                     A POPULAR TREATISE ON GEMS.
forming roundish masses ; at other times the interposition of some foreign substance (gas or vapor) has renderetl it porous, cellular or vesicular, giving rise to clrusy cavities. These cavities are often empty, but have occasionally been filled by other minerals, when the rock is named amygda-loidal, from the almond-like shape of the inclosed masses.
Many of the above external forms appear also in the amorphous solid minerals, in which no trace of individual parts, and consequently of internal structure, is observable. They are not unfrequently disposed in parallel or concen­tric layers, of uniform or distinct colors; and may assume spherical, cylindrical, stalactitic, and other appearances.
Pseudomorphism.—When the substance of one mineral assumes the external form of some other mineral, it is named a,pseudomorph. In some named incrusting pseudomorplis the original crystal is covered by a rough or drusy surface of the second mineral, frequently not thicker than paper. Occasionally the first crystal has been removed, and noth­ing but the shell remains ; or the cavity has been filled by a distinct mineral species, or a crystalloid, as it may be named, forming an exact representation of the original, but of a different substance.
More commonly the new mineral substance has gradually expelled the old, and replacing it, as it were, atom by atom, has assumed its exact form. In other cases not the whole substance of the original crystal, but only one or more of its elements, has been changed, or the whole matter has remained, but in a new condition. Thus arragonite crys­tals have been converted into calc-spar, the chemical com­position of both being identical; or gaylussite has been changed into calc-spar, andnlusite into C3ranite, by the loss of certain elements. On the other hand, anhydrite be­comes gypsum, red-copper ore malachite, by addition of new matter. Or the elements are partially changed, as
Ch. 1: Form of Minerals Page of 515 Ch. 2: Minerals: Physical Properties
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