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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:
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