We request 4 days of beamtime on ENGIN-X to measure the influence of relative grain size on stress partitioning between the constituent phases in two phase polycrystalline aggregates as they undergo plastic yielding. Our experiments will be performed on four calcite (CaCO3) + halite (NaCl) samples with the same composition but which have been fabricated by us so that the halite grain size is approximately the same in each sample while the calcite grain size is varied over three orders of magnitude. Our objectives are (1) to monitor directly the influence that relative grain size has on elastic strain (and hence stress) partitioning in polymineralic geological materials, and (2) to examine how changes in relative grain size impact on the way that the deformation within the strong phase (calcite) is accommodated.