The successful parameterization of the volume thermal expansion in the Mg,Fe olivine solid-solution series stimulated us to carry the analysis further in view of the orthorhombic symmetry of the olivines and thus augment the available thermophysical data. To this purpose we studied the three axial expansivities, αa,b,c, on the basis of the same body of data and the same model as used for our parameterization of the volume expansion.
The results are applied to various thermophysical parameters that possess a significant dependence on αa,b,c. In particular, the temperature variations of the axial adiabatic and isothermal moduli (a), the axial thermodynamic Grüneisen parameters (b), and the axial isothermal Anderson-Grüneisen parameters (c) are obtained for both forsterite (Fo) and fayalite (Fa). This allowed us to also study the response of αa,b,c axial expansivities to pressure.
Since Fo and Fa are isomorphous, it is striking that the axial thermophysical properties of Fo are distinctly less anisotropic than those of Fa. Whereas in Fo, both the axial compressibilities and the axial expansivities follow the same sequence, i.e. βT,b > βT,c > βT,a and αb > αc > αa, respectively, this expectation is violated in Fa where αc> αa > αb above 400 K. It is shown that the different anisotropies of the expansivities and hence of the α-dependent thermophysical properties can be related to differing structural responses to heating.