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twinning, variety, feldspar and common

ALBITE, an important member of the feldspar group of minerals. It stands at one end of the albite-anorthite series of triclinic feldspars (see FELDSPAR Game). It is a sodium aluminum silicate, Na Al SW., and is often called "soda feldspar' It has perfect basal cleavage and also cleaves easily parallel to the brachypinacoid. It is brittle, breaking with an uneven to conchoidal fracture. Its hardness is 6 to 6.5, and specific gravity about 2.63. Its usual color is white, whence its name (from "albus," white), but it is occasionally gray, or tinted with blue, green or red. The variety peristerite shows a delicate blue iridescence, similar to the "change of colors° of moonstone, which is also sometimes a variety of albite. Cleavelandite is a common lamellar variety, named in honor of the eminent mineralogist, Dr. P. Cleaveland, who died in 1858. Albite crystals present a great variety of forms, some of the simpler of which are quite similar to those of the monoclinic orthoclase, with which albite is often associated in parallel growths and intergrowths such as perthite. Twinning is even more common in albite than in ortho clase, and their analogy is shown by the occur rence of Carlsbad, Baveno and Manebach twins. Several other laws of twinning are, however, followed by albite, notably those known as the "albite law' and the "pericline law.' Both of

these types are very common and often mani fest themselves by the polysynthetic twinning lamellae which are so characteristic of the pla gioclase feldspars. Albite often occurs in tabu lar crystals and embedded masses in which this twinning is revealed by striations on the basal plane. Probably the most striking occurrence of albite is at Amelia, Va., this locality pro ducing large groups of tabular crystals, each over a foot in length. It usually occurs in granite or gneiss, and less frequently in the crystalline schists. It is found but rarely in volcanic rocks and in limestones. Many of the most highly prized gem minerals, such as topaz, beryl and tourmaline occur in albitic granite, while albite is often a guide mineral to colum bite, allanite and other rarer minerals. It is also an essential constituent of dioryte. There are many noteworthy localities in Switzerland, the Tyrol, Cornwall and elsewhere in Europe, while it abounds throughout the Atlantic Coast States, and is found in especially attractive specimens on amazonstone in Colorado.