Mineral description : Cordierite

Cordierite from Evje, Norway.
Cordierite from Stakholmen.

Mineral class Silicates : Cyclosilicate : Andalusite - Sillimanite - Kyanite group.
Chemical formula Mg2Al4Si5O18
Crystal system Orthorombic.
Habitus Short prismatic crystals, commonly twinned, or massive grainy aggregates.
Cleavage Distinct, fracture conchoidal.
Hardness 7-7.5
Density 2.60-2.66
Color Usually gray, but nuances in brown and blue is also common. Often strongly pleochroic. Transparent to translucent.
Streak Colorless.
Luster Vitreous.
Occurance Formed in igneous and contact metamorphic rocks (for example gneiss).
Notes Easily mistaken for quartz. Distinguished from quartz by heating in a flame. Cordierite is much easier made to glow than quartz.
Fusible on thin edges in a flame.
Locations A very common mineral in Sweden. It's a component in manySwedish schists, especially in central Sweden.
  • Falu mine. Up to dm sized yellowish brown to gray concretions in chlorite schist.
  • Riddarhyttan (esp. Källfallsgruvan). As yellowish brown concretions in schist.
  • Mellanfjärden, Hälsingland. As big clear crystals in quartzite bedrock.
  • Hudiksvall. As a black iron rich variety.
  • Nyköping area. Beautiful bllue concretions.

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Last changed : 1998-03-07