DETERMINATION OF VALENCE STATES FOR THE MINERALS

Recent analyses of the oxidation states of redox-sensitive elements in minerals show that certain elements, such as Mn, increase their valence states as Earth evolves, from compounds dominated by Mn²⁺ to those dominated by Mn⁴⁺. To investigate this and other related systematics, the oxidation states of most of the elements in the IMA mineral list has been estimated and recorded as a field within the RRUFF.info/IMA database. In general, if a compound consists of anions that are only bonded to cations, and cations that are only bonded to anions, then the chemical formula should charge balance and oxidation states are relatively straightforward to estimate, e.g. fayalite, Fe²⁺₂SiO₄. However, valence states are not so obvious in structures with anion-anion bonds, such as in pyrite, FeS₂, or cation-cation bonds as in argentopyrite Ag¹⁺Fe₂S^2-₃, largely because simple metrics, such as provided by Pauling’s rules, have not been developed for these cases.

To estimate the valence states, all minerals with reported crystal structures in the American Mineralogist Crystal Structure Database have been examined with procrystal electron density analyses to determine the coordination of the constituent atoms and provide a guide to evaluate the nature of their bonds. In addition, since bond lengths correlate with valence states, a database of over 200 million interatomic separations has been compiled to provide additional guidance. These data have been added to the RRUFF.info/IMA database and are summarized, along with the analyses, in this presentation.