The volcanic island of Fuerteventura (Canary Islands, Spain) offers the opportunity to investigate aqueous alteration in Mars-like environments. As on Mars, landscapes on Fuerteventura are typified by mafic volcanic landforms, minimal precipitation, strong winds, and minimal or absent vegetation. In this study, we perform reflectance spectral and geochemical analysis of near-surface basaltic materials from Fuerteventura's Gair & iacute;a caldera, as well as samples from a nearby arroyo. Tephra, outcrop rock, and soil-like material exhibit variations in color, spectral properties, mineralogy, and major oxides. Visible/near-infrared (VNIR) spectra of orange/light-brown materials have higher reflectance values and much stronger features attributed to phyllosilicates (including H2O and Al-OH bands near 1.41-1.45, 1.91-1.92, 2.21, and 2.76 & micro;m characteristic of montmorillonite in caldera and arroyo samples, plus shoulder features near 1.38 and 2.17 & micro;m and a band at 2.70 & micro;m indicative of kaolinite/halloysite in arroyo samples) compared to black/brown materials. Additionally, several of the highly altered samples contain spectral bands due to calcite at 2.33, 2.53, 3.36, 3.47, and 3.97 & micro;m. Major oxide data reveal similar distinctions between lighter orange/tan (altered) versus darker (unaltered) samples. Lighter and orange-colored samples show elevated Al2O3 and depleted Fe2O3T, MgO, CaO, and Na2O, as well as higher chemical index of alteration (CIA) values, overall characteristic of water-soluble cation release (and secondary clay formation) during incipient-to-intermediate chemical weathering of basalt. Gair & iacute;a weathering trends inform phyllosilicate formation in arid volcanic settings broadly. Of particular interest is the chemical alteration of basalt to montmorillonite and kaolinite/halloysite taking place in warm but water-limited desert conditions, suggesting the potential for clay formation in analogous (warm but relatively dry) paleoenvironments on early Mars.