The unit-cell dimensions and crystal structure of sillimanite at various pressures up to 5.29 GPa have been refined from single-crystal X-ray diffraction data. As pressure increases,
a and
b decrease linearly, whereas
c decreases nonlinearly with a slightly positive curvature. The axial compression ratios at room pressure are β
a:β
b:β
c=1.22:1.63:1.00. Sillimanite exhibits the least compressibility along
c, but the least thermal expansivity along
a (Skinner et al. 1961; Winter and Ghose 1979). The bulk modulus of sillimanite is 171(1) GPa with K′=4 (3), larger than that of andalusite (151 GPa), but smaller than that of kyanite (193 GPa). The bulk moduli of the [Al1O
6], [Al2O
4], and [SiO
4] polyhedra are 162(8), 269(33), and 367(89) GPa, respectively. Comparison of high-pressure data for Al
2SiO
5 polymorphs reveals that the [SiO
4] tetrahedra are the most rigid units in all these polymorphic structures, whereas the [AlO
6] octahedra are most compressible. Furthermore, [AlO
6] octahedral compressibilities decrease from kyanite to sillimanite, to andalusite, the same order as their bulk moduli, suggesting that [AlO
6] octahedra control the compression of the Al
2SiO
5 polymorphs. The compression of the [Al1O
6] octahedron in sillimanite is anisotropic with the longest Al1-OD bond shortening by ~1.9% between room pressure and 5.29 GPa and the shortest Al1-OB bond by only 0.3%. The compression anisotropy of sillimanite is primarily a consequence of its topological anisotropy, coupled with the compression anisotropy of the Al-O bonds within the [Al1O
6] octahedron.
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