In general, Andesite is the product of repeated heating and crystallization of rocks which indicates that this process on Mars was likely more prolonged than we had thought. Andesite is an extrusive igneous rock with intermediate chemistry and is composed largely of plagioclase . magnetite and quartz with smaller mixtures of olivine and orthoclastics . Andesite has a rough surface texture with a blackish-brown and occasionally greenish color. The size distribution of the grains or crystalline structures can provide important clues about the nature of the cooling and crystallization process.
Bottom line: Most geologists expected the surface rocks to be basalt as that seems to be prevalent throughout the Solar System. The presence of andesite, while not alarming, is certainly interesting and does point to a prolonged period of heating, crystallization and rock making on Mars.