Quantifying lithophilicity, chalcophilicity and siderophilicity

Abstract

The Goldschmidt classification of elements into “lithophile”, “chalcophile” and “siderophile” on the basis of geochemical preferences was devised to explain the distribution of elements through a differentiating planet, under conditions of high temperature, low bulk oxygen content and a range of pressures. Applying the concept to crustal materials is useful, if it is accepted that some elements classify very differently under low-temperature, high-oxygen conditions. A more nuanced, empirical ten-step scale of geochemical preferences has been constructed for educational use, as well as a thermochemically based quantification of preferences on the basis of two parameters which correspond respectively to “siderophile versus compound-forming” and “lithophile versus chalcophile”. Attempting to reconcile the empirical scale with the thermochemical one reveals several interesting discrepancies which nevertheless can be rationalised, and some predictive character for new types of compounds which should be sought as minerals. Overall, “lithophile” corresponds to “hard acid” in the sense of Pearson, “siderophile” corresponds to “electronegative and soft”, while “chalcophile” corresponds to “electropositive and soft”.