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Mineralogical Society of America

Figure 1.7

Click on each of the environments below.

High pressure
Low temperature

Low pressure
Low temperature

Low pressure
High temperature

The minerals kyanite, andalusite, and sillimanite all have the same formula, Al₂SiO₅. The difference between them is the number of oxygen atoms that surround one of the Al sites, shown here as the polyhedron that is somewhat transparent. There are oxygen atoms at the corners of all the polyhedra shown. In kyanite, there are 6 oxygens around each Al³⁺. In andalusite, the Al³⁺ atoms are 5-coordinated, and in sillimanite, each Al³⁺ is surrounded by only 4 oxygens. These differences reflect the pressure and temperature conditions under which each mineral forms.

Figure 1.1

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