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 Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate. ...

( Na Al Si₃ O₈) and

anorthite Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is Ca Al2 Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth but abundant on the Moon. Mineralo ...

( CaAl₂Si₂O₈). The albite:anorthite molar ratio of oligoclase ranges from 90:10 to 70:30. Oligoclase is a high sodium feldspar crystallizing in the triclinic system. The

Mohs hardness The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by ...

is 6 to 6.5 and the specific gravity is 2.64 to 2.66. The

refractive indices In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

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. Oligoclase is a common mineral in the more silica-rich varieties of igneous rock and in many metamorphic rocks.

Name and discovery

The name oligoclase was given by

August Breithaupt Johann Friedrich August Breithaupt (May 16, 1791 – September 22, 1873) was a German mineralogist and professor at Freiberg Mining Academy in Freiberg, Saxony. Biography He was born in Probstzella. He received his doctorate at the Universities ...

in 1826 from the grc, όλίγος, oligos, little, and grc, κλᾶν, 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 by J. J. Berzelius in 1824, and was named by him soda-spodumene (Natron-spodumen), because of its resemblance in appearance to

spodumene Spodumene is a pyroxene mineral consisting of lithium aluminium inosilicate, Li Al( Si O3)2, and is a source of lithium. It occurs as colorless to yellowish, purplish, or lilac kunzite (see below), yellowish-green or emerald-green hiddenite, pri ...

Occurrence

Perfectly colorless and transparent glassy material found at

Bakersville, North Carolina Bakersville is a town in Mitchell County, North Carolina, United States. The population was 464 at the 2010 census. It is the county seat of Mitchell County. History In prehistoric times, local mica deposits were extensively mined by Native Ame ...

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 Orthoclase, or orthoclase feldspar (endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles ...

, as a constituent of plutonic igneous rocks such as granite,

syenite Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger prop ...

, and diorite. It occurs in porphyry and diabase dikes and sills as well as in the volcanic rocks andesite and trachyte, and in mugearite 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, and

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

in veins in granite at

Arendal Arendal () is a municipality in Agder county in southeastern Norway. Arendal belongs to the region of Sørlandet. The administrative centre of the municipality is the city of Arendal (which is also the seat of Agder county). Some of the nota ...

in Norway. The distinctive texture of

rapakivi granite Rapakivi granite is a hornblende-biotite granite containing large round crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase). The name has come to be used most frequently as a textural term where it implies plagioclase ...

is due to oligoclase rims on orthoclase phenocrysts. Oligoclase is also found in

metamorphic rocks Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

formed under transitional greenschist to

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flak ...

facies conditions.

Schiller iridescence

Some examples are called moonstone and show Schiller iridescence due to the presence of

exsolution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wo ...

lamellae on cooling in the peristerite

miscibility gap A miscibility gap is a region in a phase diagram for a mixture of components where the mixture exists as two or more phases – any region of composition of mixtures where the constituents are not completely miscible. The IUPAC Gold Book defines ...

, ~An₅-An₁₈.

References

* Hurlbut, Cornelius S.; Klein, Cornelis, 1985, ''Manual of Mineralogy'', 20th ed., Wiley,
Webmineral data

Gemstones Tectosilicates Sodium minerals Calcium minerals Feldspar Triclinic minerals {{silicate-mineral-stub