Abstract
Hemimorphite, with the chemical formula Zn4Si2O7(OH)2·H2O, contains two different types of structurally bound hydrogen: molecular water and hydroxyl. The elastic properties of single-crystal hemimorphite have been determined by Brillouin spectroscopy at ambient conditions, yielding tight constraints on all nine single-crystal elastic moduli (Cij). The Voigt–Reuss–Hill (VRH) averaged isotropic aggregate elastic moduli are KS (VRH) = 74(3) GPa and μ (VRH) = 27(2) GPa, for the adiabatic bulk modulus and shear modulus, respectively. The average of the Hashin–Shtrickman (HS) bounds are Ks (HS) = 74.2(7) GPa and and μ (HS) = 26.5(6) GPa. Hemimorphite displays a high degree of velocity anisotropy. As a result, differences between upper and lower bounds on aggregate properties are large and the main source of uncertainty in Ks and μ. The HS average P wave velocity is VP = 5.61(4) km/s, and the HS S-wave velocity is VS = 2.77(3) km/s. The high degree of elastic anisotropy among the on-diagonal longitudinal and pure shear moduli of hemimorphite are largely explained by its distinctive crystal structure.
Highlights
The influence of hydrogen, or “water”, on the elastic properties of minerals is a topic of great current interest for possibly identifying hydrous phases in the deep crust and mantle from seismic models for these regions
From the measured Brillouin frequency shifts, ∆νB, velocity v in a given crystallographic direction i is derived from the equation for symmetric platelet geometry [12]: vi =
The nine single-crystal elastic moduli of hemimorphite have been measured at ambient conditions
Summary
The influence of hydrogen, or “water”, on the elastic properties of minerals is a topic of great current interest for possibly identifying hydrous phases in the deep crust and mantle from seismic models for these regions. The effect of water on the elastic properties of minerals can depend greatly on details of the crystal structure into which hydrogen is incorporated, and on whether hydrogen is present in the form of hydroxyl OH− or as molecular water. Ringwoodite and garnet can change elastic moduli significantly [1,2], in contrast to the olivine-chondrodite group [3], which varies less, with a similar water content. These differences are considered to be caused by different incorporation mechanisms of hydrogen into their structures. Hemimorphite, Zn4 Si2 O7 (OH)2 ·H2 O, a hydrated alteration product of willemite, Zn2 SiO4 , is interesting, in that it contains both molecular H2 O and
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