metamorphic
Staurolite (Fairy Cross)
Staurolite (Fe2+2Al9Si4O23(OH))
Hardness: 7-7.5 (Mohs scale), Color: Reddish-brown to brownish-black, Luster: Vitreous to dull, Crystal Structure: Monoclinic (pseudo-orthorhombic), Cleavage: Distinct in one direction, Specific Gravity: 3.7 to 3.8
- Hardness
- 7-7
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Physical properties
Hardness: 7-7.5 (Mohs scale), Color: Reddish-brown to brownish-black, Luster: Vitreous to dull, Crystal Structure: Monoclinic (pseudo-orthorhombic), Cleavage: Distinct in one direction, Specific Gravity: 3.7 to 3.8
Formation & geological history
Formed through regional metamorphism of aluminous sediments (shales and mudstones) in medium-grade metamorphic rocks like mica schists and gneisses. Geological age varies by location, often associated with Paleozoic mountain-building events.
Uses & applications
Primarily used as an index mineral to determine pressure and temperature of metamorphic rocks; used in jewelry as 'fairy stones,' and occasionally as an abrasive due to its hardness.
Geological facts
Staurolite frequently forms 'twinned' crystals that intersect at 60 or 90 degrees, creating natural cross shapes. In folk lore, these were called 'Fairy Crosses' and carried for good luck or protection.
Field identification & locations
Identify in the field by its distinct prismatic or cross-shaped habits and its resistance to weathering, often standing out in relief from softer schist matrix. Common locations include Fannin County (Georgia, USA), Stuart (Virginia, USA), and parts of Brazil and Russia.
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