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.

Keywords

Compressibility Bulk Modulus Compression Ratio Kyanite Sillimanite 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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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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