Metarhyolite

Metarhyolite is rhyolite, a silica-rich volcanic rock, that has undergone metamorphism. Its originally fine-grained or glassy felsic material recrystallizes into quartz, feldspar, and mica, producing a harder rock that may develop foliation while sometimes preserving volcanic textures.

Because rhyolite is chemically similar to granite, metarhyolite shares much of its mineralogy and is generally pale, grey to pink or greenish. With strong deformation it can grade into a quartzofeldspathic schist or gneiss.

Metarhyolite is common in ancient volcanic-sedimentary sequences and greenstone belts, where felsic lavas and tuffs have been buried and altered.

Metarhyolite forms when rhyolitic lavas, tuffs, and ash-flow deposits are subjected to heat and pressure during regional or contact metamorphism.

The fine groundmass and any volcanic glass recrystallize into an interlocking mosaic of quartz, alkali feldspar, and mica, while original phenocrysts of quartz and feldspar may survive as relict crystals. Deformation can produce schistosity, flattened textures, or stretched features.

Metarhyolite occurs in Precambrian greenstone belts, ancient volcanic arcs, and metamorphosed terranes worldwide, often interlayered with metabasalt, metasediments, and other metavolcanic rocks.

Identify metarhyolite by its pale, felsic composition combined with metamorphic recrystallization: a hard, fine-grained, quartz-feldspar-rich rock, sometimes with relict volcanic phenocrysts or flow features.

It is hard (about 6-7) and light in color, and may show faint foliation or a flattened fabric not present in fresh rhyolite. Relict quartz 'eyes' (rounded phenocrysts) are a useful clue.

Look-alikes include fresh rhyolite (which retains crisp volcanic textures and lacks metamorphic foliation), felsic gneiss (coarser, banded), and metasandstone (clastic, grain-supported rather than volcanic).

Metarhyolite is used as crushed stone and aggregate, and ancient peoples valued fine-grained felsic metavolcanic rock for stone tools because of its hardness and conchoidal fracture.

In economic geology, metarhyolite in greenstone belts and volcanic arcs is associated with volcanogenic massive sulfide (VMS) deposits and gold mineralization, making it an exploration target.

Scientifically, metarhyolite helps date and correlate ancient volcanic episodes and reconstruct the history of volcanic terranes. It is not a gem material, though some attractive varieties are used ornamentally.