Olivines - in particular the solid-solution between forsterite (Mg2SiO4) and fayalite (Fe2SiO4) - are important rock-forming minerals on Earth, also occurring in meteorites and interstellar dust. Olivines form the major component of Earth's upper mantle and the point at which their structure becomes unstable with respect to denser minerals marks the upper boundary of the “transition zone” between the upper and lower mantle. The thermal expansion of fayalite (Fe2SiO4) is unusual and not well understood at present. The expansion coefficients of two of the axes of the crystal have a "normal" temperature dependence, tending to zero as T goes to 0 K and becoming almost constant at high T; however, for the other axis the expansion coefficient is roughly constant between about 80 K and 1100 K. The purpose of our experiment is to understand the structural basis of these differences in behaviour