Oligoclase

Oligoclase is a plagioclase feldspar near the sodium-rich end of the series, between albite and andesine. Its name, from Greek for "little breaking," alludes to the belief that its cleavage was less perfect than albite's.

It is a common rock-forming mineral but is best known to gem enthusiasts as the host of sunstone, a variety whose orange-to-coppery glittery sheen (aventurescence) comes from tiny reflective inclusions of copper or hematite platelets. Oligoclase also forms peristerite, which displays a bluish schiller similar to moonstone due to fine albite-oligoclase intergrowths.

In typical rocks it appears as pale gray, white, or greenish grains and is widespread in granitic and intermediate igneous and metamorphic rocks.

Oligoclase crystallizes from sodium-rich igneous magmas and is common in granite, granodiorite, syenite, and many gneisses and schists. As a plagioclase it forms within the continuous albite-anorthite solid solution, its calcium content reflecting the parent melt.

Sunstone-quality oligoclase forms where the cooling crystal traps oriented platelets of copper or iron oxides that produce aventurescent flash; notable sunstone localities include Norway, India, Canada, and parts of Russia (the labradorite-type sunstone of Oregon is a different feldspar species). Peristerite forms by exsolution as albite and oligoclase unmix during slow cooling, producing the bluish sheen.

Oligoclase is a feldspar of Mohs 6-6.5 with two good cleavages near 90 degrees, vitreous luster, and white streak. Body color is usually white, gray, or greenish; sunstone oligoclase shows metallic orange-copper glints, and peristerite shows a bluish glow.

Fine albite twinning striations may be visible on cleavage surfaces, marking it as plagioclase rather than potassium feldspar. The aventurescent sparkle of sunstone is diagnostic when present.

Separating oligoclase from albite or andesine by eye is difficult and generally needs optical or chemical analysis, since they differ chiefly in their sodium-to-calcium ratio.

Oligoclase's gem varieties give it ornamental value: sunstone is cut into cabochons, beads, and faceted stones for its sparkling aventurescence, and peristerite is used as a moonstone-type gem for its blue sheen.

As a rock-forming plagioclase, oligoclase is important in petrology and the classification of igneous and metamorphic rocks, and feldspar from such rocks contributes to industrial feldspar supplies used in ceramics and glass.

In metaphysical traditions sunstone is associated with vitality, warmth, and optimism, while peristerite shares moonstone's intuitive associations; these are traditional beliefs rather than proven effects. Jewelry use is appropriate given its hardness, with care taken for cleavage.