Reversed experiments (P H2O = 1 kbar; Co-CoO or WM buffer) on the kirschsteinite (CaFeSiO 4 )-fayalite (Fe 2 SiO 2 ) join outline a miscibility gap below approximately 1041 degrees C. Experimental results are compatible with a simple, symmetric Margules model having W G of 21866+ or -143J. We treat kirschsteinite as an end-member with the implicit assumption that Ca-Fe mixing occurs only in M2 site of the olivine structure. Evidence has been adduced to show that olivines with more Ca 2 SiO 4 component than in CaFeSiO 4 can occur, but for those olivines there appears to be a strong partitioning of Ca into M2 site; thus kirschsteinite may serve as an effective end-member. Least-squares refinement of unit-cell parameters show that all three cell edges and the unit-cell volume vary linearly with composition in fayalite-kirschsteinite olivines. Based on the experimental data of Bowen et al. (1933) and an assumption of ideality in the liquid, the enthalpy of melting of kirschsteinite has been estimated at 106.1+ or -10.6 kj.--Modified journal abstract.