Glaucophane Schist Identification Guide
Identify glaucophane schist (blueschist) by its blue-gray foliated amphibole fabric, high-pressure origin, and how it differs from greenschist.
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What Glaucophane Schist Looks Like
Glaucophane schist — better known as blueschist — is a foliated metamorphic rock whose distinctive blue to blue-gray and lavender color comes from the sodium amphibole glaucophane. It has a schistose (foliated) fabric of aligned bladed/needle-like amphibole crystals, giving a silky to vitreous luster and a streaky, lineated appearance. It often contains accessory garnet, epidote, lawsonite, white mica, or red-brown spots, set against the blue matrix.
Key visual cues
- Unusual blue to blue-gray / lavender body color (rare among rocks)
- Foliated, schistose fabric with aligned acicular amphibole
- Silky sheen on cleavage surfaces
- Possible red garnet or pale epidote/lawsonite grains in a blue groundmass
Step-by-Step Field ID Checklist
- Note the blue color. A genuinely blue metamorphic rock strongly suggests blueschist.
- Confirm foliation. Aligned mineral fabric and a tendency to split along it.
- Look for amphibole. Bladed/needle-like crystals defining the foliation.
- Test hardness. Glaucophane is ~6 (scratches glass with effort).
- Spot accessories. Red garnet, pale epidote, or lawsonite confirm the high-pressure assemblage.
- Consider the tectonic setting. Subduction-zone terrains (mélanges) are the home of blueschist.
Diagnostic Tests
- Color: the blue-gray glaucophane color is the headline diagnostic — few other common rocks are blue.
- Mohs hardness: glaucophane ~6 — scratches glass; harder than the soft green chlorite of greenschist.
- Foliation/cleavage: strong schistosity; amphibole shows the typical two cleavages at ~56°/124° in larger grains.
- Streak: grayish-blue to white.
- Density: moderately high (~3.0–3.2), reflecting high-pressure minerals.
- No acid reaction.
Common Look-Alikes and How to Tell Them Apart
- Greenschist: the lower-pressure cousin — green (chlorite/actinolite/epidote) rather than blue. Color and mineralogy separate them; both are foliated metamorphic rocks but record different P-T conditions.
- Blue-gray slate/phyllite: finer-grained, with a slaty/silky cleavage and no visible blue amphibole; lacks garnet/lawsonite assemblage.
- Sodalite/lapis-bearing rock: blue but from feldspathoids, not amphibole; lapis fizzes for pyrite-free tests differently and has a different setting.
- Kyanite-bearing schist: kyanite is blue but forms bladed crystals with very different hardness anisotropy (4.5 along, 6.5–7 across) within a micaceous schist, not a glaucophane matrix.
- Amphibolite: dark green-black hornblende rock, not blue and typically less schistose.
Where Glaucophane Schist Is Found
Blueschist forms under high-pressure, low-temperature conditions unique to subduction zones, where oceanic crust is dragged deep and rapidly. Its presence is a key indicator of fossil subduction. Classic localities include the Franciscan Complex of California, the Alps, Japan, and Greece (Syros, Cyclades). Look in accretionary mélanges and exhumed subduction-zone terrains.
Frequently asked questions
How can you tell if a rock is glaucophane schist?
Look for a foliated metamorphic rock with a distinctive blue to blue-gray color from glaucophane amphibole, often with red garnet or pale epidote/lawsonite, a silky sheen, and a hardness around 6. Its subduction-zone setting confirms it.
What does glaucophane schist look like?
It looks like a blue to lavender-gray foliated rock made of aligned bladed amphibole crystals, sometimes speckled with red garnet or pale epidote, with a silky to glassy luster.
Why is glaucophane schist called blueschist?
Because the sodium amphibole glaucophane gives the rock its characteristic blue color, making blueschist a convenient common name.
Glaucophane schist vs greenschist: what's the difference?
Blueschist forms at high pressure and low temperature in subduction zones and is blue, while greenschist forms at lower pressure and is green from chlorite and actinolite. Color and mineralogy reflect different metamorphic conditions.
What does blueschist tell geologists?
Blueschist forms only under the high-pressure, low-temperature conditions of subduction zones, so finding it indicates that a region once experienced subduction.
Glaucophane Schist identified by the community
Recent Glaucophane Schist specimens identified with Rock Identifier.
