The aluminous pyroxene, fassaite, occurs in two small tabular bodies within mafic plutonites of the Boulder Batholith near its north-east margin twelve miles east of Helena, Montana. First described by Knopf & Lee (1957), the bodies are contact-metasomatized limestone septa, now magnesian-tactites, consisting chiefly of fassaite, spinel, garnet, vesuvianite, and clintonite. Less common minerals include pargasite, diopside, wollastonite, sphene, perovskite, anorthite, forsterite, calcite and chlorite. Some twenty-five microprobe analyses of the fassaite show it is variable in composition and largely consists of the components CaMgSi2O6 (53–83 per cent), CaAl2SiO6 (7–25 per cent), CaFeAlSiO6 (8–28 per cent), and CaTiAl2O6 (0–7 per cent). The stoichiometry generally requires that most of the iron is ferric, consistent with Mössbauer data taken on a typical sample. If fassaite analyses from these and other contact metamorphic rocks are plotted on a triangular diagram with Ca(Mg,Fe)Si2O6, CaAl2SiO6 and CaFeAlSiO6 as end-members, the distribution of points offers no positive evidence for a solvus gap between fassaite and diopside as proposed by Ginzburg (1969). The most aluminous fassaites occur with spinel-clintonite ± grossular and have 25 per cent of the Si replaced by Al, making them true polymorphs of a garnet (i.e. Gr42And23Pp35). No unusual cation ordering is detected in these fassaites by single-crystal X-ray photographs or Mössbauer measurements.