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From core to atmosphere, the oxidation states of elements in a planet shapeits character. Oxygen fugacity (fO$_2$) is one parameter indicating theselikely oxidation states. The ongoing search for atmospheres on rocky exoplanetsbenefits from understanding the plausible variety of their compositions, whichdepends strongly on their oxidation states -- and if derived from interioroutgassing, on the fO$_2$ at the top of their silicate mantles. This fO$_2$must vary across compositionally-diverse exoplanets, but for a given planet itsvalue is unconstrained insofar as it depends on how iron (the dominantmultivalent element) is partitioned between its 2+ and 3+ oxidation states.Here we focus on another factor influencing how oxidising a mantle is -- afactor modulating fO$_2$ even at fixed Fe$^{3+}$/Fe$^{2+}$ -- the planet'smineralogy. Only certain minerals (e.g., pyroxenes) incorporate Fe$^{3+}$.Having such minerals in smaller mantle proportions concentrates Fe$^{3+}$,increasing fO$_2$. Mineral proportions change within planets according topressure, and between planets according to bulk composition. Constrained byobserved host star refractory abundances, we calculate a minimum fO$_2$variability across exoplanet mantles, of at least two orders of magnitude, dueto mineralogy alone. This variability is enough to alter by a hundredfold themixing ratio of SO$_2$ directly outgassed from these mantles. We furtherpredict that planets orbiting high-Mg/Si stars are more likely to outgasdetectable amounts of SO$_2$ and H$_2$O; and for low-Mg/Si stars, detectableCH$_4$, all else equal. Even absent predictions of Fe$^{3+}$ budgets, generalinsights can be obtained into how oxidising an exoplanet's mantle is.

A mineralogical reason why all exoplanets cannot be equally oxidising