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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.
73
small crystals investing the surface of a large crystal or of some othes body.
The minute or cryptocrystalline minerals form similar aggregates. In the globular or the oolitic, the minute crys­tals often appear to radiate from a centre, or form concen­tric crusts. Somewhat similar are the stalactites and sta­lagmites, in which the mineral, especially rock-salt, lime­stone, chalcedony, opal, limonitc, has been deposited from a fluid dropping slowly from some overhanging body. In this case the principal axis of the figure, generally a hollow tube, is vertical, while the individual parts are arranged at right angles to this direction. In other cases the mineral has. apparently been deposited from a fluid mass moving slowly in a particular direction, which may be regarded as the chief axis of the figure, while the axes of the indi­vidual crystals may assume a different position.
By far the largest masses of the mineral kingdom have, however, been produced under conditions in which a free development of their forms was excluded. This has been the case with the greater portion of the minerals compos­ing rocks or filling veins and dykes. The structure of these masses on the large scale belongs to geology, but some varieties of the texture visible even in hand specimens may be noticed. The individual grains or masses have seldom any regular form, but appear round, long, or flat, according .to circumstances, and as each has been more or less checked in the process of formation. Even then, however, a cer­tain regularity in the position of the parts is often observ­able, as in granite, in which the cleavage planes, and con­sequently the axes of the felspar crystals, are parallel. Where these grains are all pretty similar in size and shape, the rock is named massive when they are small, or granular when they are larger and more distinct. Sometimes the rock becomes slaty, dividing into thin plates; or concretionary,
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Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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