Thermal expansion and magnetostriction in fayalite, Fe2SiO4

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橄榄石（Olivines）——尤其是镁橄榄石（forsterite，Mg₂SiO₄）与铁橄榄石（fayalite，Fe₂SiO₄）之间形成的固溶体——是地球上重要的造岩矿物，同时也广泛存在于陨石与星际尘埃之中。橄榄石是地球上地幔的主要组成成分，当其结构相较于更致密矿物发生失稳的临界位置，即为上地幔与下地幔之间「过渡带（transition zone）」的上边界。目前，铁橄榄石（Fe₂SiO₄）的热膨胀行为较为反常，尚未得到充分阐释。该晶体的两条晶轴的热膨胀系数呈现「常规」的温度依赖性：当温度趋近0 K时系数趋于零，在高温下则趋近于恒定值；然而对于其余一条晶轴而言，其热膨胀系数在约80 K至1100 K区间内大致保持恒定。本实验的研究目的，正是阐明上述热膨胀行为差异的结构成因。