Kyanite

Kyanite, andalusite and sillimanite turn into heat-resistant mullite when fired, lining the furnaces that make steel, glass and cement.

Industrial minerals · No. 74

• Not on a critical list

Three minerals with the exact same formula turn into the heat-proof lining without which no steel furnace could run.

Chart: Illustration: Kyanite

Where it comes from

Chart: Map of where it comes from

These minerals form deep in metamorphic rock under different heat and pressure. Fired, they become mullite, which stays solid above 1,500 C.

Three things to know

1. Kyanite, andalusite and sillimanite share one formula, Al₂SiO₅, and become mullite when heated, holding shape above 1,500 C.
2. Ninety per cent of US kyanite-mullite output goes into refractories, and 60-70% of that is used by iron and steel.
3. The average value of US kyanite exports rose every year, from $369 per tonne in 2021 to $510 in 2025.

The journey

1. 1 Metamorphic rockAluminous schist, gneiss or mineral sands
2. 2 Open pitOre mined from surface workings
3. 3 ConcentratingFlotation and magnetic separation
4. 4 CalciningFired into stable high-alumina mullite
5. 5 RefractoriesFirebrick and castables for steel furnaces

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