We propose to study the low temperature structural and magnetic changes in three clinopyroxene phases FeGeO3, CaNiSi2O6 and CaCoSi2O6. All three phases order antiferromagntically, a single magnetic transition is observed for CaNiSi2O6 and CaCoSi2O6, whilst two transitions are found in FeGeO3, the lower temperature transition is to a magnetoelectric phase. In conjuction with this study of the magnetism in these phases, we shall derive detailed thermoelastic parameters that are lacking in the mineralogical and geophysical literature. The temperature dependence of the crystal structure will allow us to further our ideas on parameterising clinopyroxene crystal structures in terms of the interactions of the connected polyhedra which has proved succesful in the study of jadeite.