We request three days of beam-time on POLARIS to investigate the influence of thermally-induced twinning on the thermal expansion behaviour of polycrystalline calcite. Experiments will be performed on four samples of Carrara marble with different grain sizes and lattice preferred orientations in order to vary the ease with which twinning can occur. As well as providing the basis for a significant improvement in methods that use calcite twinning to estimate the magnitude of the stresses experienced by naturally deformed rocks, the results will allow us to generalize previous observations of this phenomena to a wider range of microstructures. As such they form part of a larger experimental programme which is designed to improve our capacity to specify the thermoelastic properties of polycrystalline geological materials from the single crystal properties of their constituent mineral phases.