Abstract: Two samples of garnet-kyanite-staurolite schist from Lukmanier, Switzerland, each contain two chemical varieties of kyanite which occur in texturally distinct areas of the rock. Type 1 form large idiomorphic crystals within an open crenulation cleavage S₃. They exhibit a systematic zonation of F₂O₃, with core values of 0.8% decreasing to 0.3% at the crystal margin. Type 2 form small, much less abundant crystals in areas between S₃ cleavage zones, and have a homogeneous distribution of 0.3% Fe₂O₃ throughout the crystal.

It is suggested that the first-nucleated crystals contain the highest core concentration of Fe₂O₃ and are the largest. A positive correlation between core Fe₂O₃ values and crystal size is interpreted as a nucleation and growth sequence. This indicates that the first crystals formed preferentially in S₃ (Type 1), with Type 2 crystals growing later outside the S₃ zones.

Concentration of kyanite in S₃ zones produces a distinct mineral banding in the rock. A mechanism for the development of metamorphic differentiation by preferred nucleation of kyanite in S₃ is proposed.