Unit-cell parameters were determined for twelve mantle omphacites, at least eight of which are nonstoichiometric. The a and b cell dimensions and unit-cell volumes for the nonstoichiometric pyroxenes are consistently lower than would be expected for an ideal jadeite-diopside solution. Extrapolation to the Ca-Eskola endmember (Ca0.5□0.5AlSi2O6) indicates a cell volume of 402 ± 5 Å3 and a calculated density of 3.29 ± 0.04 g/cm3 for this component. An X-ray structure refinement of an omphacite from the Bellsbank kim-berlites, South Africa, revealed that vacancies are required in the structure in order for occupancies to be consistent with the mineral composition. Combined with electron-channeling results, the X-ray data indicate that the vacancies are largely constrained to the M2 site. The presence of vacancies does not appear to significantly affect the size of the M2 polyhedron under ambient temperature and pressure, although the Ml polyhedron of the vacancy-bearing pyroxene is substantially smaller than that predicted from endmember values.