Rocksday

Sillimanite

A fibrous aluminum silicate that forms only at very high metamorphic temperatures.

Sillimanite specimen
Photo: Robert M. Lavinsky, CC BY-SA 3.0, via Wikimedia Commons

How to identify sillimanite

  • Fibrous or long, slender prisms.
  • Silky luster on fibrous masses.
  • Found in high-grade gneiss and schist.

Often confused with

Where it's found

  • Brandywine Springs, Delaware
  • Sri Lanka
  • India

Uses

High-temperature ceramics.

Rock tumbling

Great for tumbling.

Did you know?

It's Delaware's state mineral, and it shares its exact formula with kyanite and andalusite.

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