Re-evaluating stoichiometric estimates of iron valence in magmatic clinopyroxene crystals

Clinopyroxene is a major rock forming mineral capable of incorporating diverse metal cations. As a consequence, clinopyroxene preserves valuable archives of magmatic processes. Understanding clinopyroxene is thus essential for understanding Earth’s wider chemical evolution. However, knowledge about the relative abundances of ferrous and ferric iron in magmatic clinopyroxene remains sparse because it is not currently possible to routinely measure the valence of iron in clinopyroxene crystals without either separating single crystals for bulk analysis or securing access to Mössbauer spectroscopy or a synchrotron radiation source to perform in situ microanalysis. This is despite magmatic clinopyroxene often containing appreciable quantities of ferric iron that will affect its stability and behaviour in currently ill-constrained ways and limit our ability to exploit its chemistry to robustly reconstruct the conditions of magma storage and evolution. Here we integrate optimised electron probe microanalysis and Mössbauer spectroscopy on endmember and single-crystal clinopyroxene samples to re-evaluate previously discredited approaches for estimating clinopyroxene ferric iron contents by stoichiometry. By ensuring that all major and minor elements in clinopyroxene crystals are measured with sufficient precision, we show that it is possible to readily obtain stoichiometric estimates of clinopyroxene ferric to total iron contents with similar precisions to those derived from Mössbauer spectroscopy (1σ ∼ 3.5% absolute). Being able to robustly determine clinopyroxene ferric iron contexts enables us to propose a new empirical scheme for assigning clinopyroxene components that explicitly accounts for ferric iron, which is primarily hosted within esseneite component (CaFe³+AlSiO₆) in clinopyroxenes dominated by quadrilateral components and aegirine component (NaFe³+Si₂O₆)