Staurolite-mica Schist Identification Guide
Identifying staurolite-mica schist by its shiny micaceous foliation studded with brown staurolite crystals, plus the famous fairy-cross test.
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What Staurolite-mica Schist Looks Like
Staurolite-mica schist is a medium- to high-grade metamorphic rock made of a shiny, foliated micaceous matrix (muscovite and/or biotite plus quartz) studded with dark reddish-brown to blackish staurolite porphyroblasts. The mica gives the rock a silvery to bronze sheen and a strong layered, wavy foliation that splits into flaky sheets. The staurolite crystals stand out as stubby, prismatic blocks, sometimes as the famous cross-shaped (cruciform) twins known as "fairy crosses."
Step-by-Step Field ID
- Confirm schistosity. Look for well-developed foliation: parallel, wavy mica layers that reflect light and tend to split into sheets.
- Spot the porphyroblasts. Hunt for hard, blocky brown crystals (staurolite) sitting in or across the mica fabric.
- Test crystal hardness. Staurolite is hard (Mohs 7-7.5) and scratches glass; the surrounding mica is soft and peels.
- Look for cross twins. Right-angle or 60-degree intergrown staurolite "fairy crosses" are diagnostic.
- Check the matrix. Abundant flaky mica that flakes off in thin elastic sheets confirms a mica schist host.
Key Diagnostic Tests
- Staurolite hardness: Mohs 7-7.5, scratches glass; resists weathering and often weathers out as loose crystals.
- Mica hardness: Mohs 2-3, peels into flexible flakes.
- Staurolite streak: colorless to gray (the crystals are dark brown but powder pale).
- Cleavage: mica shows one perfect basal cleavage; staurolite shows poor cleavage and stubby prismatic form.
- Texture: porphyroblastic over a foliated schistose groundmass.
Common Look-Alikes
- Garnet-mica schist: garnets are rounded, equant, glassy red dodecahedra; staurolite is brown, prismatic/elongate, and may form crosses.
- Andalusite or kyanite schist: andalusite tends pinkish and square in cross-section (chiastolite shows a dark cross internally); kyanite forms blue blades with two very different hardnesses. Staurolite is brown and harder/blockier than kyanite blades.
- Plain mica schist (no porphyroblasts): lacks the hard brown crystals.
- Hornfels: non-foliated, fine-grained, lacks the flaky schistosity.
Where It Is Found
Staurolite-mica schist forms during regional metamorphism of aluminous (pelitic) sedimentary rocks at medium grade. Classic localities include the Appalachian belt (notably Georgia, the Carolinas, Virginia), New England, the Alps, and Scotland. Weathered fairy-cross staurolites are famously collected in Georgia and around Taos, New Mexico.
Frequently asked questions
What does staurolite-mica schist look like?
A shiny, flaky, foliated mica rock (silvery muscovite or bronze biotite) studded with hard, dark reddish-brown blocky staurolite crystals, sometimes intergrown as cross-shaped fairy crosses.
How can you tell staurolite from garnet in a schist?
Staurolite is brown, prismatic or elongate and may form cross twins; garnet is glassy red and forms rounded equant grains. Both are hard (~7), but their shape and color differ.
What is a fairy cross?
A fairy cross is a naturally cross-shaped (cruciform) twin of staurolite, formed when two crystals intergrow at right angles or at about 60 degrees.
How hard is staurolite in the schist?
Staurolite is Mohs 7-7.5 and scratches glass, far harder than the surrounding mica (Mohs 2-3), so it often weathers out as durable loose crystals.
Staurolite-mica Schist identified by the community
Recent Staurolite-mica Schist specimens identified with Rock Identifier.
