Perthite

Perthite is not a single mineral species but a texture: an intergrowth in which lamellae, strings, or patches of sodium-rich feldspar (albite) are interleaved with potassium-rich feldspar (orthoclase or microcline). The name comes from Perth, Ontario, Canada.

At high temperatures, alkali feldspar can hold both potassium and sodium in one homogeneous crystal. As the crystal cools, the two components separate, or exsolve, into alternating layers, producing the characteristic streaky, often iridescent appearance.

The scale of the intergrowth varies from features visible to the naked eye (macroperthite) down to those only seen under the microscope (microperthite) or by X-ray (cryptoperthite). When the host is sodium feldspar with potassium lamellae, it is called antiperthite.

Perthite forms by exsolution during slow cooling of an originally homogeneous alkali feldspar. It is characteristic of plutonic igneous rocks such as granite, syenite, and pegmatite, where the slow cooling allows sodium and potassium to migrate and segregate into distinct lamellae.

The coarseness of the perthite texture reflects cooling history: slowly cooled deep-seated rocks develop coarse, visible perthite, while rapidly cooled volcanic feldspars (sanidine, anorthoclase) preserve a homogeneous, unexsolved structure or only cryptoperthite. Perthitic feldspar is abundant in granite pegmatites worldwide, including those of Ontario, Norway, and the southwestern United States.

Look for fine, wavy, streaky banding or a watery, milky sheen on a single feldspar crystal, often pink or cream colored, with hardness 6-6.5 and a white streak. The two interleaved feldspars give a characteristic flame- or string-like pattern on cleavage surfaces.

Perthite can resemble plain orthoclase, but the subtle striping and sometimes bluish schiller reveal the intergrowth. Unlike plagioclase, it does not show continuous fine albite-twin striations across the whole crystal; instead the lamellae are patchy or string-like. A hand lens helps reveal the texture.

As a feldspar, perthitic rock is used in the ceramics and glass industries as a source of alkalis and alumina. Large clean perthite crystals from pegmatites have historically been quarried for these purposes.

Some perthite and related feldspar shows attractive iridescence and is cut as ornamental stones or moonstone-like cabochons. Its main scientific significance, however, is as a recorder of cooling history: geologists read perthite coarseness to infer how slowly a granitic body cooled at depth.