The formula for gypsum is CaSO4(H2O)2.  In reality there are exceptions to the formula, in some cases one element is partially replaced by traces of another.  In this case the usual substitions will be the replacement of the calcium (Ca) atom with either the strontium (Sr) atom or the sodium (Na) atom. Major element chemistry (by ICP-AES) is similar for rosette types T and L, but the concentration of Ca is significantly less than that expected in stoichiometric gypsum. Trace abundances of Sr and Na have also been detected; Type L rosettes have higher Sr and Na concentrations than those from Type T rosettes.  Crystals comprising type T and L rosettes show irregular, concentric zoning under epi-fluorescence.  Furthermore, preliminary analyses using proton-induced x-ray spectrometry (u-PIXE) reveal that the inner, dark-blue fluorescing zone of crystals from both Type T and Type L rosettes have significantly higher Sr concentrations than the outer, light-green fluorescing zone.

    Oxygen and sulfur isotope analyses of gypsum show variations between the different sites that have been sampled.  Within each site, the different rosette types have different isotopic composition.  Nonetheless, based on current data, there appears to be no systematic correlation between rosette type and location.

    The stratigraphic distribution and preliminary geochemical analyses of the gypsum rosettes, in conjunction with previous ground-water data for Agassiz clays , suggest that gypsum formation was associated with lowering of the piezometric surface, followed by infiltration and ponding of meteoric water.  Isotopic data from the three study sites further suggest an organic sulfur or sulfide origin for the sulfate, and in situ formation of the sulfate.