Metapelite (Sillimanite Zone)
“Mud transformed at the highest metamorphic grade”
Sillimanite-zone metapelite represents the highest grade of regional metamorphism that a mud-derived rock can reach before beginning to melt. At this stage, all lower-grade index minerals have been consumed, and sillimanite — the high-temperature polymorph of Al₂SiO₅ — dominates alongside garnet and alkali feldspar.
sillimanite, garnet, alkali feldspar, cordierite, biotite, quartz, spinel (rare)
Scottish Highlands (Barrow's sillimanite zone), Broken Hill (Australia), Kerala (India), Adirondacks (USA), Limpopo Belt (Southern Africa)
6-7
The transition from kyanite to sillimanite in metapelites marks one of the most important boundaries in metamorphic geology — it signals the rock has crossed roughly 650°C, approaching the melting point of granite.
Test Your Knowledge
Sillimanite, kyanite, and andalusite all share the same chemical formula but form under different conditions. What is their shared formula?
Related Specimens
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