The average chemical composition of the Biwabik Iron-Formation and of its principal members is reasonably well known from several previous studies. In this paper the mineral compositions of the formation and its subdivisions are calculated from these chemical data.
Because certain components of the formation, such as ferrous iron, occur in oxide, carbonate, and silicate minerals, there is no unique way of assigning such elements to normative minerals. Moreover, because the mineralogy varies with metamorphic grade, the degree of metamorphism must be arbitrarily set. The present calculations were designed to include only the iron minerals magnetite, siderite, minnesotaite, stilpnomelane, and hematite, which are known from field studies to be the chief constituents of the main part of the formation that is essentially unmetamorphosed.
According to the plan adopted here, the calculated mineral composition of the Biwabik Formation by weight percent is: quartz = 31.9, iron silicates = 28.0, magnetite = 18.4, combined carbonates = 16.3, hematite = 4.6, and other minerals = 0.8. These values cannot be compared with the over-all modal composition of the unmetamorphosed formation, because that has not been determined. Several lines of evidence, however, suggest that the calculated mineral abundances do not deviate far from the actual abundances.