oligoclase

Oligoclase
Oligoclase from near Crown Point, Essex County, New York, United States of America
General
Category	plagioclase, feldspar, tectosilicate
Chemical formula	(Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) (% Anorthite) is between 10%-30%
Identification
Color	usually white, with shades of grey, green or red.
Mohs Scale hardness	6 to 6.5
Specific gravity	2.64 to 2.66
Refractive index	nα=1.533 - 1.543, nβ=1.537 - 1.548, and nγ=1.542 - 1.552
Birefringence	1st order

Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈). The albite : anorthite molar ratio ranges from 90 : 10 to 70 : 30.

Oligoclase is a high sodium feldspar crystallizing in the triclinic system. The Mohs hardness is 6 to 6.5 and the specific gravity is 2.64 to 2.66. The refractive indices are: nα=1.533 - 1.543, nβ=1.537 - 1.548, and nγ=1.542 - 1.552. In color it is usually white, with shades of grey, green or red.

The name oligoclase was given by August Breithaupt in 1826 from the Greek oligos, little, and kasein, 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.

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 found as reddish cleavage masses in gneiss at Tvedestrand in southern Norway; this presents a brilliant red to golden metallic glitter, due to the presence of numerous small scales of hematite oriented within the feldspar structure.

Oligoclase occurs, often accompanying orthoclase, as a constituent of plutonic igneous rocks such as granite, syenite, and diorite. It occurs in porphyry and diabase dikes and sills as well as in the volcanic rocks 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. The distinctive texture of rapakivi granite is due to oligoclase rims on orthoclase phenocrysts. Oligoclase is also found in metamorphic rocks formed under transitional greenschist to amphibolite facies conditions.

Some examples show Schiller iridescence due to the presence of exsolution lamellae on cooling in the peristerite miscibility gap, ~An₅-An₁₈.

References

• This article incorporates text from the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.
• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
• Webmineral data
• Mineral galleries
• Mindat