Abstract. By means of in situ observation, the nucleation and growth of triclinic, pseudo-orthorhombic and pseudo-hexagonal CaA12Si208 were investigated and their relative stabilities established. Pseudo-hexagonal and pseudo-orthorhombic CaA12Si208 nucleate prior to anorthite and grow in a supercooled anorthite melt before the latter appears. They either dissolve or transform to anorthite once the latter nucleates and starts to grow. Corundum and mullite also nucleate metastably prior to anorthite in this melt. The liquidus temperatures of the four metastably nucleated phases were determined precisely by in situ measurements of the transformation temperatures from a polyhedral to a rounded morphology. On the basis of the supercooling thus determined, the growth rate versus supercooling relations were measured and analysed for the respective phases. The growth rate of the stable phase is one order of magnitude higher than those of any metastable phases. However, the growth rates of the three CaAlzSi2Os polymorphs normalized by the viscosities are nearly the same.
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