ABSTRACT

A method is described for routine calculation of mineralogical compositions from chemical analyses of sedimentary rocks. Although designed for the particular mineral suites found in the Florena Shale (a mid-continent Permian argillaceous unit containing calcite, dolomite, chert, and clay as major constituents), it can be adapted with slight modification for use with a wide variety of sedimentary rock types. After restating the bulk analysis in terms of molecular ratios, TiO₂, Na₂O, P₂O₅, SO₃, and S are disposed of as rutile, albite, apatite, gypsum, and pyrite. Next, all of the CaO and CO₂ and as much MgO as is required, are used to form calcite and dolomite. The balance of the MgO and all of the K₂O are then used to form illite and either sericite or chlorite, depending on the relative amounts of MgO and K₂O. Residual Fe₂O₃ and Al₂O₃ are computed as montmorillonite and residual SiO₂ is regarded as chert. Results of computation compare well with X-ray and insoluble residue data and the method is judged to provide an estimate of the proportions of clay, dolomite, calcite, and chert accurate to within 5 per cent.