Relationships between crystal structure, bonding and thermal stability of amphiboles

Abstract

The complexities in structure and chemical composition of the amphiboles and the wide range of their occurrence suggest that the amphiboles are potential index minerals for the physical conditions of their formation. Hydrothermal stability studies of several amphibole endmembers have demonstrated that the FeMg₋₁ substitution produces a wide spread in thermal stability. The crystal structure, upon substitution, responds to the differences in cation size and site occupancy, but the changes are small.

In order to correlate the observed stability variation with the observed differences in crystal structure of amphiboles, structure parameters as well as calculated bond strengths, Madelung site energies, and average bond overlap populations obtained from Extended Hückel Molecular Orbital (EHMO) calculation for different cation sites, were examined. Among the examined structural parameters, calculated Madelung site potentials, and bond strength, only the parameters involving bonds between the M(1)- and M(3)-cations with OH show higher correlations with the thermal stability as compared to those of the M(2)- and M(4)-cations. This reflects the dehydration nature of the amphibole break-down reaction, since the OH bonding with seems important in controlling the thermal stability.

Infrared absorption spectra of amphiboles show the fine structure of the hydroxyl group. The frequency of the absorption band is related directly to the strength of the OH bond. Positive correlations exist between thermal stability and OH-stretching frequency for different amphibole end-members at different temperatures. For epidote minerals, as well as for muscovite and phlogopite, high OH-stretching frequencies also correlated with the minerals having higher thermal stability. These correlations indicate that the thermal stability of many hydrous minerals may be significantly related to the localized force field around the OH bond.

The crystal structure of grunerite Klein No. 9A has been refined and compared with grunerite Klein No. 1 (Finger, 1969) and cummingtonite (Ghose, 1961; Fisher, 1966). The results show that the substitution of Fe for Mg into the cummingtonite-grunerite structure not only enlarges the octahedral layer but also affects the size of the T(1) and T(2) tetrahedra and thus increases strain on the octahedral and tetrahedral linkage. The high thermal expansion of the octahedral layer and the negligible effect of heating on the tetrahedral layer substantially increase the strain on the structure. This may also explain why the Fe-rich end member decomposes at a lower temperature than its Mg-analogue, the structure of which can accommodate the build-up of strain with increasing temperature.