Citation: Thompson, R. M.; Downs, R. T. (2004). Model pyroxenes II: Structural variation as a function of tetrahedral rotation model hi-P protopx at 9.98 GPa after Yang et al. (1999) with O3A-O3A-O3A angle = 147.8, O3B-O3B-O3B angle = 220.8, and model oxygen radius = 1.307 Note: Atoms Mg, Si, and O are assigned to atomic positions for convenience. American Mineralogist, 89, pp. 614-628.