metamorphic
Blue schist (likely glaucophane schist)
Glaucophane schist; chiefly glaucophane amphibole with mica, quartz, and possible lawsonite/epidote
AI-generated identification · may be incorrectTypically blue to blue-black, with a fine-to-medium crystalline, schistose texture and weak to strong foliation; glaucophane has Mohs hardness about 6–6.5 and a vitreous to pearly luster. The specimen appears dark, granular, and foliated, though the image alone cannot confirm mineral identity.
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Physical properties
Typically blue to blue-black, with a fine-to-medium crystalline, schistose texture and weak to strong foliation; glaucophane has Mohs hardness about 6–6.5 and a vitreous to pearly luster. The specimen appears dark, granular, and foliated, though the image alone cannot confirm mineral identity.
Formation & geological history
Forms when basaltic oceanic crust undergoes high-pressure, relatively low-temperature metamorphism in subduction zones, commonly during the Mesozoic or younger tectonic cycles. It is characteristic of blueschist-facies rocks.
Uses & applications
Used mainly as a geological teaching and collecting specimen; rarely used as dimension stone because its foliation can make it split or weather unevenly. Attractive blue pieces have modest lapidary and display value.
Geological facts
Its blue color commonly comes from sodium-rich glaucophane, making it an important indicator of ancient subduction. Associated minerals can include lawsonite, jadeite, epidote, garnet, and quartz.
Field identification & locations
Common in subduction complexes such as California, Japan, Greece, Turkey, New Zealand, and the Alps; identify it by blue-black color, foliation, and dense metamorphic texture. A field test or thin section is needed to distinguish it from blue amphibolite or altered basalt.
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