Sugar cane bagasse and straw are vast, inexpensive renewable resources that can be used as solid fuels as well as feedstocks for production of materials, chemicals, and advanced biofuels. However, industrial valorization of bagasse and straw can be hampered by the mineral impurities commonly present in the biomass. In this work, we use synchrotron X-ray computed microtomography (mu CT) and scanning X-ray microfluorescence (mu XRF) to observe and analyze mineral particles in sugar cane bagasse and straw. On the basis of the microscale analyses, we suggest possible mechanisms that contribute to the incorporation of extraneous inorganic matter into biomass, especially soil debris. Our analyses discriminate between (1) mineral particles having a characteristic range of size (circa 5-27 mu m) found at surfaces as well as inside biomass particles, (2) the contribution of water that keeps larger mineral particles adhered to the biomass, and (3) soil aggregates that are adhered to the surfaces of straw, with no equivalent observed in bagasse. By focusing on microscale observations and proposition of mechanisms, we provide a framework to rationalize the high variability of inorganic contents in agricultural residues such as sugar cane bagasse and straw.