The chemistry and cell parameters of omphacites and related pyroxenes

~SUMMARY. In an attempt to correlate the chemical compositions and cell sizes of omphacites and related pyroxenes, the cell dimensions of fifty-five analysed pyroxenes have been determined, or taken from the literature. Twenty-two f the chemical analyses are new, nineteen of them being done by ~electron microprobe. Approximately two-thirds of the total number of analyses may be considered first class, the remainder are of doubtful or unknown quality. Cell parameters, determined by X-ray powder diffraction methods, have errors of-i:o-1 % for the majority of samples, although for some samples taken from the literature rrors are unknown. The majority of methods of recalculating omphacite analyses into their end-member molecules are unsuitable for correlation of cell constants with chemistry, mainly due to the impossibility ofgraphical representation f more than three end-member molecules, and to the non-stoichiometry of these molecules. Using a modification of Tr6ger's (1962) method of recalculating chloromelanite analyses the present analyses have been recalculated into the diopside-jadeite-acmite and diopside-jadeite-hedenbergite molecules and compared with their determined cell parameters. Because of the grada-tions in all parameters between these end-member molecules, determination f compositions based on the cell parameters (a, b, c, vol, or/3) can only be made within wide limits. However, using a metho