Are the thermodynamic properties of natural and synthetic Mg2SiO4-Fe2SiO4 olivines the same?

Abstract

Abstract It is not known if the thermodynamic behavior of some minerals and their synthetic analogues are quantitatively the same. Olivine is an important rock-forming substitutional solid solution consisting of the two end-members forsterite, Mg2SiO4, and fayalite, Fe2SiO4. We undertook the first heat capacity, CP, measurements on two natural olivines between 2 and 300 K; nearly end-member fayalite and a forsterite-rich crystal Fo0.904Fa0.096. Their CP(T) behavior is compared to that of synthetic crystals of similar composition, as found in the literature. The two natural olivines are characterized by X-ray powder diffraction and 57Fe Mössbauer spectroscopy. The X-ray results show that the crystals are well crystalline. The Mössbauer hyperfine parameters, obtained from a fit with two Fe2+ quadrupole split doublets, are similar to published values measured on synthetic olivines. There are slight differences in the absorption line widths (i.e., FWHM) between the natural and synthetic crystals. CP (2 to 300 K) is measured by relaxation calorimetry. The CP results of the natural nearly end-member fayalite and published values for two different synthetic Fa100 samples are in excellent agreement. Even CP resulting from a Schottky anomaly and a paramagnetic-antiferromagnetic phase transition with both arising from Fe2+ are similar. There are slight differences in the Néel temperature between the natural 63 K and synthetic ~65 K fayalites. This is probably related to the presence of certain minor elements (e.g., Mn2+) in the natural crystal. The third-law entropy, S°, value is 151.6 ± 1.1 J/(mol·K). CP behavior of the natural forsterite, Fo0.904Fa0.096, and a synthetic olivine, Fo90Fa10, are in excellent agreement between about 7 and 300 K. The only difference lies at T < 7 K, as the former does not show Debye T3 behavior, but, instead, a plateauing of CP values. The S° value for the natural forsterite is 99.1 ± 0.7 J/(mol·K).