Oligoclase

Oligoclase is a rock-forming mineral belonging to the plagioclase division of the feldspars. In chemical composition and in its crystallographical and physical characters it is intermediate between albite (NaAlSifO,) and anorthite (CaAliSiaOs), being an isomorphous mixture of three to six molecules of the former with one of the latter. It is thus a sodalime feldspar 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. ??, little, and ??, 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 6 and the sp. gr. 2.65-2.67. In color it is usually whitish, with shades of grey, green or red.

Perfectly colorless and transparent glassy material found at Bakersville, North Carolina has occasionally been faceted as a gemstone. Another variety more frequently used as a gemstone is the aventurine-feldspar 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 ?thite enclosed in the feldspar.

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.