metamorphic
Glaucophane / Blueschist
Glaucophane (Na2Mg3Al2Si8O22(OH)2)
Hardness: 6 (glaucophane component); Color: Distinctive blue-gray to lavender-blue with tan/yellowish weathered crust; Luster: Vitreous to pearly or silky; Cleavage: Good prismatic in two directions; Crystal Structure: Monoclinic
- Hardness
- 6 (glaucophane component)
- Luster
- Vitreous to pearly or silky
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Physical properties
Hardness: 6 (glaucophane component); Color: Distinctive blue-gray to lavender-blue with tan/yellowish weathered crust; Luster: Vitreous to pearly or silky; Cleavage: Good prismatic in two directions; Crystal Structure: Monoclinic
Formation & geological history
Formed under high-pressure, low-temperature metamorphic conditions in subduction zones. This specimen was found near 39.1°N, 122.9°W in Lake County, California, which sits directly within the Franciscan Complex—a world-famous subduction zone terrain rich in blueschist and glaucophane.
Uses & applications
Geological research and indicator mineral for subduction zone tectonics; highly sought after by rockhounds and specimen collectors for its distinct blue coloration.
Geological facts
Glaucophane is named from the Greek 'glaukos' (blue-green) and 'phainos' (appearing). It is the characteristic diagnostic mineral that defines the blueschist metamorphic facies.
Field identification & locations
Look for a striking slate-blue to grayish-blue interior revealed under a yellowish or pale brown weathered skin. Found commonly throughout northern California Coast Ranges in Franciscan mélange deposits.
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