Constructing mineralogical composition from geochemical composition in sediments

Raimon Tolosana-Delgado

Geoscience Center Göttingen, Dept. of Sedimentology/Environmental Geology

Talk by Raimon Tolosana-Delgado in MN 16, on Tuesday, 18th November 2008, 16:15h

The determination in the lab of the mineral composition of a sediment is a more complex issue than the determination of its geochemical composition. Therefore, one could, in princicple, try to estimate the mineral composition from the geochemical one, by using the mineral stochiometric formulae. However, that is not feasible, due to the sheer number of existing mineral species, and the lack of a fixed stochiometry for the vast majority of them. In crystalline rocks (specially igneous), one can use well-studied relations of equilibrium and mineral stability to estimate the "normative" mineral composition. That is not possible for sediments and sedimentary rocks, because sediments are not necessarily in equilibrium. As an alternative, we propose the following purely algebraic method. First, the analyst must choose which mineral species are present, and their representative stochiometry. Second, by using generalized inversion, one looks for the set of "possible" mineral compositions. Finally, one of these possible compositions is chosen, satisfying the fact that components must be positive, and minimizing some weighted "error". The "constructed" mineral compositions tend to accomodate all mineral species given. As a by-product, one can also find which mineral combinations could be "algebraically reactive", only according to their compatible stochiometric compositions.