Migmatite Identification Guide
A field guide to identifying Migmatite by its mixed light-and-dark folded banding, granitic leucosome layers, and high-grade metamorphic texture.
Read the full Migmatite encyclopedia entry →
What Migmatite Looks Like
Migmatite is a high-grade metamorphic rock that looks "half-melted." It combines dark metamorphic host rock with light, granite-like material that segregated into swirling, folded, or veined layers. The result is a dramatic, contorted banding.
- Color: Strong contrast of pale (white/pink/gray) and dark (gray/black) bands.
- Components: Light leucosome (quartz + feldspar), dark melanosome (biotite, amphibole, garnet), and intermediate mesosome (the parent gneiss/schist).
- Texture: Coarse-grained, often crenulated, folded, or "ptygmatic" (intricately crumpled light veins).
- Luster: Non-metallic; micas may glitter.
Step-by-Step Field ID Checklist
- Look for two contrasting materials in one rock: light granitic streaks woven through darker rock.
- Trace the banding: Genuine migmatite shows folded, swirling, or pinch-and-swell light veins, not straight sedimentary layers.
- Check grain size: Coarse enough to see individual quartz, feldspar, and mica grains with the eye or a loupe.
- Identify the light layer: Glassy gray quartz and blocky pink/white feldspar = leucosome.
- Identify the dark layer: Flaky biotite, needle-like amphibole, or red garnet = melanosome.
- Confirm it is metamorphic/igneous-mixed, not sedimentary, using the tests below.
Key Diagnostic Tests
- Mohs hardness: Bulk rock scratches glass because of abundant quartz/feldspar (~6–7).
- Acid: No fizz with dilute HCl (silicate, not carbonate) — distinguishes it from banded marble.
- Cleavage/fracture: Breaks across grains; micas show one perfect cleavage; feldspars show blocky cleavage.
- Density: ~2.6–2.8 g/cm3.
- Component test: Feldspar (blocky, hardness 6) vs quartz (glassy, no cleavage, hardness 7).
Common Look-Alikes and How to Tell Them Apart
- Gneiss: Has banding too, but the layers are continuous and parallel; migmatite layers are folded, lobed, and clearly look partially molten/segregated.
- Granite: Uniform speckled texture with no distinct light/dark layering; migmatite is layered.
- Banded marble: Soft (3), fizzes in acid; migmatite does not react.
- Layered gabbro: Dark overall, lacks pale granitic leucosome veins.
- Folded schist: Finer-grained and dominated by aligned mica, without coarse granitic melt pods.
Where It Is Found
Migmatites form in the deep roots of mountain belts at very high temperature (partial melting). They are common in Precambrian shields and ancient orogenic terranes — e.g., the Canadian Shield, Scandinavia, Scotland, and the Adirondacks.
Frequently asked questions
How do you identify migmatite?
Look for a single rock that mixes light granite-like veins (quartz and feldspar) with darker metamorphic rock, arranged in folded, swirling, or lobed bands. It is coarse-grained, scratches glass, and does not fizz in acid.
What is the difference between migmatite and gneiss?
Gneiss has continuous, roughly parallel light and dark bands. Migmatite shows folded, contorted, or pod-like light granitic segregations indicating partial melting, giving it a more chaotic, half-melted look.
What does migmatite look like?
It looks like dark rock injected and folded together with pale granitic streaks, often in dramatic swirls — a high-contrast, crumpled banded rock.
Is migmatite igneous or metamorphic?
It is transitional: a high-grade metamorphic rock that has partially melted, so it contains both metamorphic and igneous (melt-derived) components.
Migmatite identified by the community
Recent Migmatite specimens identified with Rock Identifier.
