The Fe2+ ions in pyroxenes occurring in basalts on surfaces of terrestrial planets are vulnerable to oxidation, particularly in magma in contact with the atmosphere; this produces Fe3+-bearing assemblages that may mask spectral features of remnant Fe2+ in pyroxenes in telescopic reflectance spectra of oxidized planetary surfaces. To assess such interferences, reflectance spectra (0.3--2.6 μm) were obtained for a variety of pyroxenes that had been heated in air at elevated temperatures. Mossbauer spectral measurements enabled the relative proportions of Fe3+ and Fe2+ ions and Fe3+-bearing phases to be determined in the reaction products. Nanophase hematite having particle diameters smaller than 10 nm obscures the pyroxene Fe2+/M2 site crystal field (CF) bands of oxidized enstatite and pigeonite. Intensification of Fe2+→Fe3+ intervalence charge transfer transitions in calcic clinopyroxenes mask pyroxene Fe2+/M1 site CF bands, as well as the Fe2+/M2 site CF bands of augites and subcalcic hedenbergites. The ferric iron derived from oxidation of Fe2+ ions may remain as structural Fe3+ ions in the oxidized calcic clinopyroxenes or exist as clusters of nanophase hematite particles having diameters of a few unit cells.