In hardness the feldspars range from 5 to 7, being usually near 6, and almost always distinctly softer than quartz. Sp. gr. varies from 2.5-2.75; lustre, from vitreous to pearly; color, from white and gray to red, brown, green, etc., but usually light. Streak, always white; rarely transparent. By exposure to the weather, feldspars gradually lose their alkalies and lime, become hydrated, and are changed to kaolin or common clay. A similar change takes place with the micas, augite, and hornblende; but these species, being usually rich in iron, make clays which are much darker colored than those derived from feldspars. The fact that the feldspars contain little or no iron undoubtedly explains their low specific gravity and light colors, as compared with the other minerals just named. The only common minerals for which the feldspars are liable to be mistaken are quartz and the carbonates. From the latter they are easily distinguished by their superior hardness and non-effervescence with acids; and from the former, by possessing distinct cleavage, by being rarely transparent, by being somewhat softer, and by changing to clay on exposure to the weather. The feldspars of greatest geological interest are five in number, and may be classified chemically as follows:— Orthoclase,—silicate of aluminum and potassium, or potash feldspar. Albite,—silicate of aluminum and sodium, or soda feldspar. Anorthite,—silicate of aluminum and calcium, or lime feldspar. Oligoclase,—silicate of aluminum and sodium, and calcium, or soda-lime feldspar. Labradorite,—silicate of aluminum and calcium, and sodium, or lime-soda feldspar. This appears like a complex arrangement, but it can be simplified. Orthoclase crystallizes in the monoclinic system, and all the other feldspars in the triclinic system. With the exception of albite, which is a comparatively rare species, the triclinic feldspars all contain less silica than orthoclase; _i.e._, are more basic. This is shown by the subjoined table giving the average composition of each of the feldspars:— SiO₂ Al₂O₃ K₂O Na₂O CaO Total. Orthoclase, 65 18 17 -- -- = 100 Albite, 68 20 -- 12 -- = 100 Oligoclase, 62 24 -- 9 5 = 100 Labradorite, 53 30 -- 4 13 = 100 Anorthite, 43 37 -- -- 20 = 100