Physics and Chemistry of Minerals

, Volume 25, Issue 1, pp 39–47 | Cite as

Compressibility and crystal structure of sillimanite, Al2SiO5, at high pressure

  • H. Yang
  • R. M. Hazen
  • L. W. Finger
  • C. T. Prewitt
  • R. T. Downs
ORIGINAL PAPER

Abstract 

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 βabc=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 [Al1O6], [Al2O4], and [SiO4] polyhedra are 162(8), 269(33), and 367(89) GPa, respectively. Comparison of high-pressure data for Al2SiO5 polymorphs reveals that the [SiO4] tetrahedra are the most rigid units in all these polymorphic structures, whereas the [AlO6] octahedra are most compressible. Furthermore, [AlO6] octahedral compressibilities decrease from kyanite to sillimanite, to andalusite, the same order as their bulk moduli, suggesting that [AlO6] octahedra control the compression of the Al2SiO5 polymorphs. The compression of the [Al1O6] 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 [Al1O6] octahedron.

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Copyright information

© Springer-Verlag Berlin Heidelberg 1997

Authors and Affiliations

  • H. Yang
    • 1
  • R. M. Hazen
    • 1
  • L. W. Finger
    • 1
  • C. T. Prewitt
    • 1
  • R. T. Downs
    • 2
  1. 1.Geophysical Laboratory, 5251 Broad Branch Road, NW, Washington, DC 20015-1305
  2. 2.Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USAUS

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