Anorthoclase

Anorthoclase is an alkali feldspar rich in sodium but containing significant potassium, with a composition straddling the boundary between albite and sanidine. It crystallizes in the triclinic system and is typically found in sodium-rich volcanic and shallow intrusive rocks.

It is most famous for two occurrences: large, glassy, gem-like crystals ejected from the lava lake of Mount Erebus in Antarctica, and the blue-iridescent feldspar that gives the Norwegian ornamental rock larvikite its shimmer.

Like other alkali feldspars it can show fine exsolution textures and a play of blue or silvery schiller, making select material attractive for ornamental and gem use.

Anorthoclase forms from sodium-rich, relatively high-temperature magmas, typically in alkaline volcanic rocks such as trachyte, phonolite, and sodic rhyolite, as well as in shallow intrusions like larvikite (a variety of monzonite). Rapid cooling near the surface preserves its high-temperature, sodium-rich structure.

The spectacular Mount Erebus crystals grow within a long-lived, convecting phonolitic lava lake and are carried up in volcanic bombs. Larvikite, mined extensively near Larvik in southern Norway, crystallized in a slowly cooled shallow intrusion, allowing fine perthitic exsolution that produces its blue schiller.

Anorthoclase is colorless to gray with a vitreous luster, hardness 6-6.5, white streak, and two cleavages near 90 degrees. It often shows a fine cross-hatched (tartan) twinning under the microscope and may display a blue or silvery schiller in hand specimen.

It is difficult to distinguish from sanidine or microcline by eye alone; chemistry and optics are needed for certainty. Context helps: large clear crystals from sodic volcanics or blue-flashing feldspar in dark larvikite point strongly to anorthoclase. Its blue schiller can mimic labradorite, but labradorite is a calcium plagioclase from gabbroic rocks.

Anorthoclase's main commercial role is as the feldspar in larvikite, a popular dimension and ornamental stone used for facing buildings, countertops, and tiles thanks to its dramatic blue-gray flash. It is sometimes marketed under trade names like Blue Pearl and Emerald Pearl granite.

Gem-quality transparent anorthoclase, including faceted Mount Erebus crystals, is a collector curiosity. It has no major metaphysical tradition of its own, though larvikite is sometimes sold as a grounding or protective stone in the crystal trade.