Sillimanite Gneiss
“Evidence of extreme crustal heat”
Sillimanite Gneiss
"Evidence of extreme crustal heat"
Sillimanite gneiss is a high-grade metamorphic rock containing the aluminum silicate mineral sillimanite, which only forms at the highest temperatures of regional metamorphism. Its presence signals that the rock reached temperatures above 650°C, placing it in the most extreme metamorphic conditions short of melting.
sillimanite, quartz, feldspar, garnet, biotite, cordierite (occasional)
Adirondacks (New York, USA), Scottish Highlands, Limpopo Belt (Southern Africa), Kerala (India), Broken Hill (Australia)
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Sillimanite is one of three minerals with the exact same chemical formula (Al₂SiO₅) but different crystal structures — along with kyanite and andalusite, they form at different pressures and temperatures, creating a geological thermometer.
Test Your Knowledge
How do the three Al₂SiO₅ polymorphs (sillimanite, kyanite, andalusite) function as a geological tool?
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