This study examined the effects of variations in mineral spatial distributions on reaction rates in porous media. Pore-scale network models were constructed to represent sandstones with anorthite and kaolinite present in various amounts and in different spatial patterns. Simulation conditions corresponded to geological sequestration of carbon dioxide. To examine scaling effects arising from pore-scale heterogeneities, reaction rates from network models were compared to rates from continuum models that use uniform concentrations. With small percentages of reactive minerals the scaling effects are large, and the effect of spatial distribution is significant. Scaling effects are largest when reactive minerals are closely clustered and oriented parallel to flow. Conversely, spatial distributions that enhance mass transport to and from reactive minerals, such as small mineral clusters or elongated clusters oriented transverse to flow, result in reaction rates that are well represented by the continuum model.