Staurolite Schist Identification Guide
How to recognize staurolite schist, a foliated metamorphic rock bearing hard brown staurolite porphyroblasts and occasional cross twins.
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What Staurolite Schist Looks Like
Staurolite schist is a foliated metamorphic rock in which dark reddish-brown to brownish-black staurolite porphyroblasts grow within a schistose matrix of mica, quartz, and often garnet or kyanite. The rock shows clear schistosity: parallel, lustrous layers that reflect light and split unevenly. The staurolite crystals are stubby, prismatic, glassy to resinous brown blocks that interrupt the foliation, and the rock often carries the cross-shaped twins prized as fairy stones.
Step-by-Step Field ID
- Check for foliation. Wavy, parallel mica-rich layers with a silvery sheen confirm a schist.
- Find the brown crystals. Look for hard, blocky, reddish-brown porphyroblasts standing proud of the matrix.
- Hardness test the crystals. Staurolite is Mohs 7-7.5 and scratches glass; the matrix mica is soft.
- Search for crosses. Right-angle or oblique twinned staurolite (fairy crosses) clinches the ID.
- Note accessory minerals. Garnet, kyanite, or sillimanite often accompany staurolite in the same rock.
Key Diagnostic Tests
- Staurolite hardness: Mohs 7-7.5, scratches glass.
- Streak: colorless to grayish white despite dark crystal color.
- Luster: vitreous to resinous on staurolite; pearly/silvery on mica.
- Cleavage: staurolite poor (one direction); prismatic habit dominant.
- Specific gravity: staurolite ~3.7-3.8, noticeably dense for its size.
Common Look-Alikes
- Garnet schist: garnet forms rounded glassy red balls; staurolite is brown, elongate, and may form crosses.
- Kyanite schist: kyanite occurs as blue-gray blades with a marked hardness difference along versus across the blade; staurolite is brown and blocky.
- Tourmaline-bearing schist: schorl tourmaline is black with striated, three-sided prisms; staurolite is brown and lacks the rounded triangular cross-section.
- Hornfels: lacks foliation entirely.
Where It Is Found
Staurolite schist develops during regional (Barrovian) metamorphism of aluminum-rich pelitic rocks at medium grade, often marking the "staurolite zone." Classic occurrences include the Appalachians (Georgia, Virginia, the Carolinas), New England, the Swiss/Austrian Alps, the Scottish Highlands, and Brittany in France.
Frequently asked questions
How can you tell if a rock is staurolite schist?
Look for a foliated, micaceous schist holding hard, dark reddish-brown blocky staurolite crystals (Mohs 7-7.5 that scratch glass), often accompanied by garnet or kyanite and sometimes cross-shaped twins.
What is the difference between staurolite schist and staurolite-mica schist?
They are essentially the same rock type; 'staurolite-mica schist' simply emphasizes that the host matrix is a mica-rich schist, which is the usual case.
Why is staurolite associated with fairy crosses?
Staurolite commonly forms penetration twins at right angles or about 60 degrees, producing natural cross shapes called fairy crosses or fairy stones.
What minerals are found with staurolite in schist?
Garnet, kyanite, sillimanite, muscovite, biotite, and quartz are typical companions, reflecting medium-grade metamorphism of aluminous sediments.
Staurolite Schist identified by the community
Recent Staurolite Schist specimens identified with Rock Identifier.
