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Critical to spectral interpretations of geologic surfaces on the Earth and Mars is an understanding of the relationship between the composition of weathered rock surfaces and whole-rock mineralogy. In this study, thermal infrared spectroscopic and remote-sensing analyses were used to determine the composition of weathered granitoid rock surfaces in the Sacaton Mountains, Arizona. A linear spectral deconvolution approach was employed to determine the mineralogies of naturally exposed, weathered surfaces and artificially exposed, fresh rock surfaces. Deconvolution results from fresh rock surfaces yield similar bulk mineralogy to results from point counting of rock slabs and thin sections. Deconvolution of weathered rock spectra indicate that compared to fresh samples, weathered surfaces are deficient in plagioclase feldspar and are enriched in clay minerals. The differential weathering of feldspar minerals impacts the interpretation of whole-rock mineralogy and the spectral classification of bedrock geology, which, for plutonic rocks, is dependent on the relative proportions of different feldspars. Deconvolutions of multispectral remote-sensing or laboratory data show a deficiency of total feldspar in the weathered rock units but lack sufficient spectral resolution for the discrimination between different feldspars. These results demonstrate that the differential breakdown and removal of primary minerals in rock surfaces due to weathering can affect the interpreted rock petrology from remote-sensing studies. The relationships observed in our granitic samples could be applied to any rock type and may be important to consider for remote sensing of Mars.