metamorphic
Glaucophane / Blue Schist
Glaucophane (Na2(Mg3Al2)Si8O22(OH)2)
Hardness: 6 (Mohs scale). Color: Blue, bluish-grey, to greyish-black. Luster: Vitreous to pearly or dull. Crystal structure: Monoclinic (amphibole group). Cleavage: Perfect in two directions intersecting at ~56° and ~124°.
- Hardness
- 6 (Mohs scale)
- Color
- Blue, bluish-grey, to greyish-black
- Luster
- Vitreous to pearly or dull
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Physical properties
Hardness: 6 (Mohs scale). Color: Blue, bluish-grey, to greyish-black. Luster: Vitreous to pearly or dull. Crystal structure: Monoclinic (amphibole group). Cleavage: Perfect in two directions intersecting at ~56° and ~124°.
Formation & geological history
Formed through high-pressure, low-temperature (HP-LT) regional metamorphism in subduction zone environments, typical of oceanic crust subduction.
Uses & applications
Primarily collected as a classic index mineral for subduction zone metamorphism (Blueschist facies). Used in educational collections and geological research.
Geological facts
Glaucophane is named from the Greek words for 'blue' and 'to appear'. It is the characteristic diagnostic mineral defining the 'blueschist' metamorphic facies. The user's location in Lake County, Northern California, is in the heart of the Franciscan Complex, world-famous for blueschist and glaucophane specimens.
Field identification & locations
Look for distinct blue to bluish-grey amphibole minerals embedded in matrix or fine-grained schistose rock, often associated with quartz, lawsonite, or garnet. Common throughout the Coast Ranges of California.
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