metamorphic
Glaucophane-bearing Schist (Blueschist)
Blueschist (Glaucophane-Lawsonite Schist)
Hardness: 5-6 (due to glaucophane contents); Color: Distinctive blue to bluish-grey or dark green; Luster: Pearly to vitreous; Crystal Structure: Monoclinic; Cleavage: Good (prismatic); Specific Gravity: 3.0-3.2
- Hardness
- 5-6 (due to glaucophane contents)
- Color
- Distinctive blue to bluish-grey or dark green
- Luster
- Pearly to vitreous
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Physical properties
Hardness: 5-6 (due to glaucophane contents); Color: Distinctive blue to bluish-grey or dark green; Luster: Pearly to vitreous; Crystal Structure: Monoclinic; Cleavage: Good (prismatic); Specific Gravity: 3.0-3.2
Formation & geological history
Formed through high-pressure, low-temperature metamorphism (HP/LT) typically in subduction zones. This occurs when oceanic crust is pushed deep into the mantle but remains relatively cool. The geological age varies depending on the tectonic site, often found in Mesozoic to Tertiary terranes.
Uses & applications
Primary use is geological research for understanding plate tectonics. Occasional use as decorative stone or in garden landscaping due to unique blue color. Collected by mineralogists for its indicator status of past subduction.
Geological facts
Blueschists provide direct evidence of ancient subduction zones; they are essentially 'fossils' of deep-earth movement. The mineral glaucophane, which gives it the blue color, is named from the Greek 'glaukos' for bluish-green and 'phanos' for appearance.
Field identification & locations
Field Identification: Look for a bluish tint in a foliated or schistose texture, often associated with darker green minerals (actinolite) or white minerals (lawsonite/quartz). Common in the Franciscan Complex of California, the Alps, and Japan. Collectors should look for distinct blue bands.
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