metamorphic
Staurolite (Fairy Cross)
Staurolite (Fe2+2Al9Si4O23(OH))
Hardness: 7 to 7.5 Mohs; 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 to 7
- Color
- reddish-brown to brownish-black
- Luster
- vitreous to dull
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Physical properties
Hardness: 7 to 7.5 Mohs; 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
Forms during regional metamorphism of clay-rich sedimentary rocks (shales) under intermediate to high pressure and temperature; typically found in mica schists and gneisses.
Uses & applications
Used in the past as an abrasive (sandblasting) due to its hardness; currently primarily used as a geologic 'index mineral' to estimate depth and temperature of metamorphic rocks; popular as a charm or collector's specimen.
Geological facts
Commonly forms 'twinned crystals' where two crystals grow across each other at 90 or 60 degrees, creating a cross shape. It is the official state mineral of Georgia, USA, and folklore suggests they are tears of fairies or cherubs.
Field identification & locations
Identify by its characteristic cross twinning and its presence within a mica schist matrix. Often found in eroded riverbeds or soil near metamorphic belts. Common locations include Fannin County (Georgia), Stuart (Virginia), and parts of the Swiss Alps.
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