Staurolite-mica Schist

Staurolite-mica schist is a foliated metamorphic rock consisting of a micaceous schist matrix studded with porphyroblasts of staurolite — stubby, dark red-brown to brownish-black crystals that grew during metamorphism.

Staurolite is famous for its cross-shaped (cruciform) twins, popularly called "fairy stones" or "fairy crosses," which weather out of the rock and are prized as natural amulets. The schist commonly also carries garnet, kyanite, and abundant mica.

It is a classic indicator of medium- to high-grade metamorphism and is studied worldwide as a marker of metamorphic conditions.

Staurolite-mica schist forms by regional metamorphism of aluminous (pelitic) shales at medium-high grade, roughly 550-650 degC and moderate pressure — the so-called staurolite zone of Barrovian metamorphism.

Staurolite crystallizes when iron- and aluminum-rich clays are heated enough to break down chlorite and produce this distinctive index mineral, growing as porphyroblasts that often poke through the surrounding mica foliation. The cruciform twins form by intergrowth of two crystals at near 60 or 90 degree angles.

Notable localities include Fannin County, Georgia (USA); Taos, New Mexico; Brittany, France; the Scottish Highlands; and Switzerland.

The diagnostic feature is blocky red-brown to dark brown staurolite crystals set in a silvery mica schist, sometimes as distinctive cross-shaped twins. The crystals are hard (Mohs 7-7.5) and resist scratching, in contrast to the soft, flaky mica matrix.

Staurolite has a dull to vitreous luster and a colorless to gray streak; its crystals are typically prismatic and stubby. The surrounding schist sparkles and splits along mica planes.

Look-alikes: garnet schist has rounded red garnet balls rather than elongate prisms; andalusite (chiastolite) schist shows square cross-sections with dark crosses inside, not raised crystals. The cruciform twins are essentially unique to staurolite.

Staurolite-mica schist has little bulk industrial use, but its staurolite fairy-cross twins are popular as natural curios, amulets, and pendants, collected and sold especially from the Appalachian localities. Staurolite is the state mineral of Georgia.

Industrially, staurolite sand recovered from heavy-mineral deposits is used as an abrasive blasting medium and foundry sand.

For geologists, the rock is a key index mineral assemblage used to map metamorphic grade, while collectors prize well-formed crosses and gem-quality clear staurolite crystals.