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OLIGOCLASE, a rock-forming mineral belonging to the plagioclase (q.v.) division of the felspars. In chemical composition and in its crystallographical and physical characters it is intermediate between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), being an isomorphous mixture of three to six; molecules of the former with one of the latter. It is thus a soda-lime felspar crystallizing in the anorthic system. Varieties intermediate between oligoclase and albite are known as oligoclase-albite. The name oligoclase was given by A. Breithaupt in 1826 from the Gr. όλίγος, little, and κλᾶν, to break, because the mineral was thought to have a less perfect cleavage than albite. It had previously been recognized as a distinct species by J. J. Berzelius in 1824, and was named by him soda-spodumene (Natron-spodumen), because of its resemblance in appearance to spodumene. The hardness is 61/2 and the sp. gr. 2.65-2.67. In colour it is usually whitish, with shades of grey, green or red. Perfectly colourless and transparent glassy material found at Bakersville in North Carolina has occasionally been faceted as a gem-stone. Another variety more frequently used as a gem-stone is the aventurine-felspar or “sun-stone” (q.v.) found as reddish cleavage masses in gneiss at Tvedestrand in southern Norway; this presents a brilliant red metallic glitter, due to the presence of numerous small scales of haematite or göthite enclosed in the felspar.

Oligoclase occurs, often accompanying orthoclase, as a constituent of igneous rocks of various kinds; for instance, amongst plutonic rocks in granite, syenite, diorite; amongst dike-rocks in porphyry and diabase; and amongst volcanic rocks in andesite and trachyte. It also occurs in gneiss. The best developed and largest crystals are those found with orthoclase, quartz, epidote and calcite in veins in granite at Arendal in Norway.  (L. J. S.)