metamorphic
Glaucophane Schist (Blueschist)
Blueschist facies metamorphic rock
Hardness: 6-6.5 for glaucophane; Color: blue, grey, or greenish-grey with dark needle-like crystals; Luster: vitreous to pearly; Crystal structure: Monoclinic (amphibole group); Cleavage: perfect in two directions at 56 and 124 degrees; Specific gravity: 3.0-3.3
- Hardness
- 6-6
- Color
- blue, grey, or greenish-grey with dark needle-like crystals
- Luster
- vitreous to pearly
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Physical properties
Hardness: 6-6.5 for glaucophane; Color: blue, grey, or greenish-grey with dark needle-like crystals; Luster: vitreous to pearly; Crystal structure: Monoclinic (amphibole group); Cleavage: perfect in two directions at 56 and 124 degrees; Specific gravity: 3.0-3.3
Formation & geological history
Formed under high-pressure, low-temperature conditions (HPLT) typically found in subduction zones where oceanic crust is pushed beneath continental crust. This specimen shows characteristic glaucophane needles in a fine-grained matrix.
Uses & applications
Primarily used for geological research to identify ancient plate tectonic boundaries. Some high-quality specimens are used in decorative stone work or as collector items.
Geological facts
Blueschist is a diagnostic marker for ancient subduction zones. The presence of the mineral glaucophane gives it its distinctive color and needle-like (acicular) crystal habit seen in this image.
Field identification & locations
Identify in the field by looking for deep blue to dark grey needle-like crystals within a foliated schistose matrix. It is commonly found in the Franciscan Complex of California, the Alps, and Japan.
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