Oligoclase Composition, Crystallization & Structure
Composition. Intermediate between albite and anorthite, chiefly near 3NaAlSi3O8.1CaAl2Si2O8.
Crystallization. Triclinic. Like albite.
Structure. Usually massive, cleavable to compact. Crystals rare.
Physical Properties. Cleavage in two directions at 86° 32’. One cleavage (parallel to base ) is better than the other, and on this parallel striation lines due to twinning are commonly to be seen. H. = 6. G. = 2.66. Vitreous to pearly luster. Color usually whitish with faint tinge of grayish green, also reddish white, etc. Translucent to opaque.
Tests. Fusible at 4-4.5. Insoluble in hydrochloric acid. To be told from albite only by a test for calcium. Briefly, the test is made as follows: Fuse powdered mineral with sodium carbonate; dissolve fusion in hydrochloric acid and evaporate to dryness, moisten residue with water and a little nitric acid, boil and then filter off insoluble silica; to filtrate add ammonium hydroxide in excess, filter off calcium oxalate upon addition of ammonium oxalate. To be positively distinguished from andesine and labradorite only by a chemical analysis or an optical examination.
Occurrence. Like albite, but not so common. Especially characteristic of the more acid igneous rocks. Found in various localities in Norway, notably at Tvedestrand, where it contains inclusions of hematite, which give the mineral a golden shimmer and sparkle. Such feldspar is called aventurine oligoclase or sunstone. Occurs in the United States at Dine St, Lawrence County, New York; Danbury, Connecticut; Bakersvile, North Carolina, etc.
Name. Derived from two Greek words meaning little and fracture.
Use. Occasionally used as an ornamental material, in the varieties sunstone and moonstone.
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