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Ch. 2: Minerals: Physical Properties

Ch. 2: Minerals: Physical Properties Page of 515 Ch. 2: Minerals: Physical Properties Text size:minus plus Restore normal size   Mail page  Print this page
PHYSICAL PROPERTIES OF MINERALS.                   95
to adamantine in flint-glass. (4..) Resinous, when the body appears as if smeared with oil, as in pitch-stone and garnet. (5.) Pearly, like mother-of-pearl, seen in stilbite, gypsum, mica. (6.) Silky, the glimmering lustre seen on fine fibrous aggregates like amianthus.
Color.—This property is not in all cases of equal value as a character. Thus some minerals are naturally colored, showing in all modes of their occurrence one determinate color, which is therefore essential, and forms a characteristic of the species. This class includes the metals, pyrites, blendes, with many metallic oxides and salts. A second class of minerals are colorless, their purest forms being white, or clear like water, as ice, calc-spar, quartz, adularia, and many silicates. But these minerals are occasionally colored—that is, accidentally tinged, sometimes from the chemical or mechanical admixture of some coloring sub­stance, as a metallic oxide, carbon, or particles of colored minerals; at other times from the substitution of a colored for an uncolored isomorphous element. The colors of these minerals therefore vary indefinitely, and can never charac­terize the species, but only its varieties. Thus, quartz, calc-spar, flu or spar, gypsum, and felspar are often colored accidentally by pigments mechanically mixed; and horn­blende, augite, garnet, and other colorless silicates, acquire green, brown, red, or black tints from the introduction of the isomorphic coloring elements.
Werner, who bestowed much attention on this portion of mineralogy, distinguished eight principal colors,—white, gray, black, blue, green, yellow, red, and brown,—each with several varieties or shades arising from intermixture with the other colors. lie also divided them into metallic and non-metallic as follows:
Ch. 2: Minerals: Physical Properties Page of 515 Ch. 2: Minerals: Physical Properties
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