Properties Depending Upon Light, Luster and Nonmetallic Luster

Luster
The luster of a mineral is its appearance due to the effect of light upon it. In general we divide minerals into three classes depending upon their luster, and nonmetallic luster. A mineral having the appearance of a metal like lead or copper is said to have a metallic luster. The term is further defined by saying that a mineral with metallic luster of a mineral can be proved by observing the color of its powder. If the powder is black or very dark in color, it means that each little particle of the mineral is still opaque to light, and therefore the mineral has a metallic luster. This test is made usually by the aid of what is called a streak plate. This consist of a piece of un glazed white porcelain upon which the mineral is rubbed so that a streak of its powder is formed upon the plate. The color of this “streak” of the mineral, as it is called, will determine its luster and also frequently will materially help in its identification. Examples of minerals with metallic luster would be, galena, PbS, with a bluish gray streak; pyrite, FeS2, with a black streak; chalcopyrite, CuFeS2, with a greenish black streak; and hematite, Fe2O3, with a dark reddish brown streak.

Nonmetallic Luster. Minerals with a nonmetallic luster are transparent to light on their thin edges. In general they are light colored, but not necessarily so. When a streak is obtained from a nonmetallic mineral, it is either colorless or very light in color. Various descriptive terms are used to further describe the appearance of nonmetallic minerals, the more common being as follows:

• Vitreous. Having the luster of glass. Example, quartz.
• Resinous. Having the appearance of resin. Example, sphalerite.
• Pearly. Having the iridescent appearance of a pearl. This is usually observed in minerals on surfaces that are parallel to cleavage planes. Example, basal plane on apophyllite.
• Greasy. Looking as if covered with a thin layer of oil. Examples, some specimens of sphalerite and massive quartz.
• Silky. Like silk. It is the result of a fine fibrous structure. Examples, fibrous malachite, serpentine, etc.
• Adamantine. Having a hard, brilliant luster like that of a diamond. It is due to the mineral’s high index of refraction (see p. 77). The transparent lead minerals,. Like cerussite and anglesite, show it.

Submetallic Luster
There is no sharp divisional line between minerals with metallic and those nonmetallic luster, and the group of minerals lying between is said to have a submetallic luster. They show a colored streak, but one which is not black or very dark in color. Examples of minerals with submetallic luster are limonite and some of the darker varieties of sphalerite.