## Abstract

The single-crystal elastic constants of natural alunite (ideally KAl_{3}(SO_{4})_{2}(OH)_{6}) were determined by Brillouin spectroscopy. Chemical analysis by electron microprobe gave a formula KAl_{3}(SO_{4})_{2} (OH)_{6}. Single crystal X-ray diffraction refinement with R_{1} = 0.0299 for the unique observed reflections (Fο > 4σ_{F}) and wR _{2} = 0.0698 for all data gave a = 6.9741(3) Å, c = 17.190(2) Å, fractional positions and thermal factors for all atoms. The elastic constants (in GPa), obtained by fitting the spectroscopic data, are C_{11} = 181.9 ± 0.3, C_{33} = 66.8 ± 0.8, C_{44} = 42.8 ±0.2, C_{12} = 48.2 ± 0.5, C_{13} = 27.1 ± 1.0, C_{14} = 5.4 ± 0.5, and C_{66} = (C_{11}-C_{12}) = 66.9 ± 0.3 GPa. The VRH averages of bulk and shear modulus are 63 and 49 GPa, respectively. The aggregate Poisson ratio is 0.19. The high value of the ratio C_{11}/C_{33} = 2.7 and of the ratio C_{66}/C_{44} = 1.6 are characteristic of an anisotropic structure with very weak interlayer interactions along the c-axis. The basal plane (001) is characterized by 0.1% longitudinal acoustic anisotropy and 0.9-1.1% shear acoustic anisotropy, which gives alunite a characteristic pseudo-hexagonal elastic behavior, and is related to the pseudo-hexagonal arrangement of the Al(O,OH)_{6} octahedra in the basal layer. The elastic Debye temperature of alunite is 654 K. The large discrepancy between the elastic and heat capacity Debye temperature is also a consequence of the layered structure.

Original language | English (US) |
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Pages (from-to) | 567-573 |

Number of pages | 7 |

Journal | Physics and Chemistry of Minerals |

Volume | 33 |

Issue number | 8-9 |

DOIs | |

State | Published - Nov 1 2006 |

## All Science Journal Classification (ASJC) codes

- Materials Science(all)
- Geochemistry and Petrology

## Keywords

- Alunite
- Crystal structure
- Elastic constants