Structure and density of molten fayalite at high pressure

The structure of molten fayalite was studied up to 7.5 GPa by means of in situ energy-dispersive X-ray diffraction. The pressure-range studied covers the fayalite-spinel-liquid triple point at 6.2 GPa. For pure molten fayalite, Fe-O coordination increases gradually from 4.8(2) at ambient pressure (P) to 7.2(3) at 7.5 GPa. Compressibility of the melt is derived from the extrapolation of the structure factor to q = 0 Å−1, enabling the determination of density as a function of P with an unprecedented P-resolution. This is a promising method to extract the equation of state of non-crystalline materials at moderate P. The link between observed structural changes and density increase and the fact that structural changes occur over a broad but limited P-range in silicate melts implies that the equation of state should not be extrapolated at P-values higher than obtained in measurements, and that a single equation of state cannot accurately describe the density evolution over the whole terrestrial mantle P-temperature (T) range. Fe-rich melts are expected to have a higher densification rate than their Mg counterparts in the 0-10 GPa range due to the increase of Fe-O coordination number. As a consequence, Fe-rich melts are more likely to be trapped at depth.