Metarhyolite Identification Guide

What Metarhyolite Looks Like

Metarhyolite is metamorphosed rhyolite — a felsic (silica-rich) volcanic rock that has been recrystallized, usually at low to medium grade. It retains the pale, fine-grained character of rhyolite but develops a metamorphic fabric: faint foliation, sericite (fine white mica) sheen, and recrystallized quartz-feldspar groundmass. Relict volcanic features such as phenocrysts of quartz and feldspar or flattened pumice/flow banding often survive.

• Color: light — white, grey, pink, tan, pale green (with sericite/chlorite)
• Luster: dull groundmass with a silky sericite sheen on foliation surfaces
• Transparency: opaque
• Texture: very fine recrystallized matrix; relict porphyritic phenocrysts of quartz/feldspar; weak schistosity or flattened fabric

Step-by-Step Field-ID Checklist

1. Note the felsic, pale color and fine grain — like rhyolite but with a metamorphic overprint.
2. Look for relict phenocrysts: rounded/embayed quartz eyes and feldspar crystals set in a fine matrix indicate a volcanic protolith.
3. Feel for a sericite sheen: silky, slightly foliated surfaces from fine white mica signal metamorphism.
4. Check for flattened textures: stretched former pumice, flow bands, or fiamme show deformation.
5. Confirm hardness: abundant quartz makes it hard and resistant.

Key Diagnostic Tests

• Hardness: ~6.5-7 overall (quartz + feldspar); scratches glass.
• Streak: white.
• Acid: inert (no carbonate).
• Density: ~2.6, typical felsic value.
• Hand lens: look for recrystallized quartz-feldspar mosaic plus fine mica (sericite) defining a weak foliation.

Common Look-Alikes and How to Tell Them Apart

• Fresh rhyolite: has a glassy or cryptocrystalline groundmass, crisp flow banding, and no sericite sheen or foliation. The silky mica sheen and a faint schistosity mark the metamorphosed version.
• Metaquartzite: nearly monomineralic quartz with a sugary granoblastic texture and no feldspar phenocrysts or volcanic relicts; metarhyolite contains feldspar and relict phenocrysts.
• Felsic schist/phyllite: more strongly foliated and recrystallized, losing volcanic relicts; metarhyolite preserves recognizable phenocrysts and is only weakly foliated.
• Tuff/metatuff: may look similar, but tuff shows fragmental (pyroclastic) textures — shards and lapilli — rather than the coherent lava texture of rhyolite.

Where It Is Typically Found

Metarhyolite is common in ancient volcanic terranes and greenstone belts, in metamorphosed island-arc sequences, and in Precambrian shields where felsic volcanics were folded and metamorphosed. It often hosts volcanogenic massive sulfide (VMS) deposits, so it is of economic interest in old mining districts.