Thermal expansion and high-temperture crystal chemistry of the Al ₂ SiO ₅ polymorphs Available to Purchase

The crystal structures of sillimanite and andalusite have been refined from intensity data collected at 25, 400, 600, 800, and 1000°C. R factors following refinement were 0.033±.002 and 0.029±.002 for the sillimanite and andalusite data sets respectively. For kyanite 2140, 1773, and 1741 reflections were measured at 25, 400, and 600°C, and the final R factors were 0.033, 0.031, and 0.036 respectively.

Unit-cell dimensions of all three polymorphs vary linearly with temperature. Although the unit-cell dimensions determined at room temperature agree within error limits with those of Skinner et al. (1961), significant deviations occur between the two data sets at elevated temperatures. All the Al octahedra exhibit considerable expansions with increasing temperature. In contrast, Al- or Si-tetrahedra in all three polymorphs remain relatively constant in size and shape as temperature is increased. Within the five-coordinated Al₂ polyhedron in andalusite the four short bonds remain relatively unchanged, whereas the longest bond, Al₂–O_c, expands considerably. The orientation of the long Al₁–O_c bonds in sillimanite and andalusite, which are more expandable than the other octahedral Al–О bonds, determines the direction of maximum unit-cell expansion. The chains of fully stretched tetrahedra (and Al₂ trigonal bipyramids in andalusite) restrict expansion in the с cell direction for these two minerals. The greater number of shared octahedral edges in kyanite, as well as the lack of continuous tetrahedral chains, results in more evenly distributed coeflicients of unit-cell expansion.

Polymorphic transitions involve major reconstructive transformations. In addition, the andalusite-sillimanite transition requires diffusive interchange of half the Si and Al₂ atoms. Consequently, although metastable coexistence of two or three polymorphs is commonly observed, coherent replacement textures are rare. The present volume-temperature data agree well with the experimentally-derived thermodynamic properties of the aluminum silicate minerals.