Evidence for kinks in structural and thermodynamic properties across the forsterite–fayalite binary from thin-film IR absorption spectra

Abstract

Infrared absorbance spectra over ∼100 to 1,800 cm−1 were collected from optically thin films of 21 samples with compositions spanning the forsterite–fayalite binary. Polarization information from previous specular reflectance data on end-members was used in tracing the peaks across the entire binary. Peak positions (νi) were constrained by Lorentzian decompositions. Fitting also constrained widths for singlet peaks but for doublets and triplets, variation in νi with composition among the constituent polarizations alters widths from intrinsic. Because film thicknesses of 0.6–1.4 μm were estimated, our band strengths are approximate; however, relative intensities should be correct. Only for a few peaks does νi vary smoothly across the entire binary; instead, distinct linear trends exist for Fe- and Mg-rich olivines. Discontinuities and kinks in νi(X) occur at X = Mg/(Fe + Mg) = 0.7 and are accompanied by a change in intensity patterns. This interesting behavior was not revealed in previous spectra of powder dispersions. The contrasting character of IR vibrations for Fe- and Mg-rich olivines is inferred to arise from structural variations because (1) frequency is related to bond length, (2) other factors affecting frequency (cation mass and probably bonding type) vary linearly across the binary, and (3) available data on unit cell parameters are consistent with distinct trends for forsterites and fayalites. Vibrational components of heat capacity (C V) and enthalpy (H) calculated from νi, were found to be slightly more negative than linear interpolations between values for forsterite and fayalite. Our computations give smaller negative excesses from ideality in H than do previous calorimetric measurements, but are equal within experimental uncertainties.