Microscopic strain, local structural heterogeneity and the energetics of silicate solid solutions

Evidence for the existence and scale of local microscopic strains in silicate minerals is reviewed. From the compositional limit of the plateau for the monoclinic reversible arrow triclinic phase transition in alkali feldspars, the microscopic strain fields around impurity atoms in framework silicates can be similar to 20-40 Angstrom in diameter. This dimension fails within the characteristic length scale of phonons in these materials, and phonon spectra should therefore be particularly sensitive to the extent and magnitude of local elastic strain effects. The most prominent feature is expected to be broadening of absorption peaks, which can now be followed quantitatively in complex spectra by autocorrelation analysis. Such strain fields contribute elastic energy to the enthalpy of a crystal and might be expected to be largely responsible for enthalpy changes accompanying cation ordering processes and solid solution formation. Comparisons of line broadening data from powder IR absorption spectra with calorimetric determinations of enthalpy for selected mineral systems confirm a close relationship between local structural heterogeneity and enthalpy variations.