Blueschist Identification Guide
A field guide to recognizing blueschist, the blue, glaucophane-rich metamorphic rock formed in subduction zones, and telling it from greenschist.
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What Blueschist Looks Like
Blueschist is a metamorphic rock with a distinctive bluish-gray to lavender-blue color caused by the sodic amphibole glaucophane. Despite the name it is often not strongly schistose; it can be fine-grained and massive with a faint foliation. Fresh surfaces show a felted or fibrous bluish matrix, frequently dotted with reddish-brown garnet, pale lawsonite or epidote, white mica, and shiny green omphacite or pale jadeitic pyroxene. Luster is dull to silky on cleavage flakes of the amphibole.
Step-by-Step Field ID Checklist
- Confirm the overall blue to blue-gray cast on a fresh (not weathered) surface.
- Look with a hand lens for needle-like blue amphibole crystals (glaucophane).
- Search for small red-brown garnet porphyroblasts and pale lawsonite/epidote.
- Check for weak foliation or lineation of the blue needles.
- Consider the geologic setting: blueschist signals high-pressure, low-temperature metamorphism.
Key Diagnostic Tests
- Mineral hardness: glaucophane is ~6; garnet ~7; the rock overall resists a steel knife.
- Streak: the rock leaves a pale bluish-gray to white streak.
- Cleavage/fracture: amphibole needles show two cleavages at ~120°; the rock breaks unevenly.
- Density: moderately heavy (SG ~3.0–3.2) due to dense high-pressure minerals.
- Acid: no reaction to dilute HCl (distinguishes from blue-gray marble or limestone).
- Color persistence: the blue is a primary mineral color, not a stain, and survives on broken faces.
Common Look-Alikes and How to Tell Them Apart
- Greenschist: green (chlorite/actinolite/epidote) rather than blue; forms at lower pressure. The two can grade into each other.
- Blue-gray slate or phyllite: much softer, splits into flat sheets, and lacks blue amphibole needles.
- Sodalite-bearing or dumortierite rocks: lack the fibrous amphibole texture and garnet.
- Eclogite: related high-pressure rock but dominated by red garnet and green omphacite with little or no blue glaucophane.
Where Blueschist Is Found
Blueschist forms in subduction zones where oceanic crust is taken to high pressure but kept relatively cool. Classic localities include the Franciscan Complex of California, the Western Alps, Greece (Syros, Sifnos), Japan, and Anglesey in Wales. Hunt for it in fault-bounded mélange terranes and ophiolite belts.
Frequently asked questions
How can you tell if a rock is blueschist?
Find a metamorphic rock with a blue to lavender-gray cast from needle-like glaucophane amphibole, often with red-brown garnet and pale lawsonite or epidote, that does not fizz in acid and is hard enough to resist a steel knife.
What does blueschist look like?
It is a fine- to medium-grained bluish-gray rock with a felted or faintly foliated texture, speckled with small garnets and pale high-pressure minerals.
Blueschist vs greenschist: what's the difference?
Color and conditions: blueschist is blue from glaucophane and forms at high pressure and low temperature, while greenschist is green from chlorite and actinolite and forms at lower pressure.
Why is blueschist blue?
Its color comes from glaucophane, a sodium-rich amphibole that crystallizes only under the high-pressure, low-temperature conditions of subduction zones.