| Oligoclase | |
|---|---|
 Oligoclase from Chihuahua, Mexico | |
| General | |
| Category | Tectosilicate minerals |
| Group | Feldspar group |
| Series | Plagioclase feldspar series |
| Formula | (Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) (%anorthite) is between 10–30% |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (sameH-M symbol) |
| Identification | |
| Color | Usually white, with shades of grey, green, or red[1] |
| Cleavage | Perfect {001}, Good {010}, Poor {110}. |
| Fracture | Uneven, sub-conchoidal |
| Mohs scale hardness | 6 to 6.5 |
| Luster | Vitreous |
| Streak | White |
| Specific gravity | 2.64 to 2.66 |
| Refractive index | nα = 1.533–1.543; nβ = 1.537–1.548; nγ = 1.542–1.552 |
| Birefringence | 1st order |
Oligoclase is a rock-formingmineral belonging to theplagioclasefeldspars. In chemical composition and in its crystallographic and physical characters it is intermediate betweenalbite (NaAlSi3O8) andanorthite (CaAl2Si2O8).[1] The albite:anorthite molar ratio of oligoclase ranges from 90:10 to 70:30.
Oligoclase is a high sodium feldspar crystallizing in thetriclinic system. TheMohs hardness is 6 to 6.5 and thespecific gravity is 2.64 to 2.66. Therefractive 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.[1]
Oligoclase is a common mineral in the moresilica-rich varieties ofigneous rock and in manymetamorphic rocks.
The name oligoclase was given byAugust Breithaupt in 1826 from theAncient Greek:όλίγος,romanized: oligos, little, andAncient Greek:κλᾶν,romanized: klân, to break, because the mineral was thought to have a less perfect cleavage than albite. It had previously been recognized as a distinct species byJ. J. Berzelius in 1824, and was named by him soda-spodumene (Natron-spodumen), because of its resemblance in appearance tospodumene.[1]
Perfectly colorless and transparent glassy material found atBakersville, North Carolina has occasionally been faceted as agemstone. Another variety more frequently used as a gemstone is theaventurine-feldspar or sun-stone found as reddish cleavage masses ingneiss atTvedestrand in southernNorway; this presents a brilliant red to golden metallic glitter, due to the presence of numerous small scales ofhematite oriented within the feldspar structure.[1]
Oligoclase occurs, often accompanyingorthoclase, as a constituent of plutonicigneous rocks such asgranite,syenite, anddiorite. It occurs inporphyry anddiabasedikes andsills as well as in thevolcanic rocksandesite andtrachyte, and inmugearite where its presence is a defining feature. It also occurs in gneiss. The best developed and largest crystals are those found with orthoclase,quartz,epidote, andcalcite in veins in granite atArendal in Norway.[1] The distinctive texture ofrapakivi granite is due to oligoclase rims on orthoclasephenocrysts. Oligoclase is also found inmetamorphic rocks formed under transitionalgreenschist toamphibolite facies conditions.
Some examples are calledmoonstone and show Schiller iridescence due to the presence ofexsolution lamellae on cooling in the peristeritemiscibility gap, ~An5-An18.
One of the iridescent varieties of oligoclase, discovered in 1925 near theWhite Sea coast by academicianAlexander Fersman, became widely known under the trade namebelomorite.[2]
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Spencer, Leonard James (1911). "Oligoclase". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 20 (11th ed.). Cambridge University Press. p. 82.