Which climate conditions will cause the fastest chemical weathering of granite sandstone and shale?

From having worked in the tropics a little, I would say that tropical conditions can be ideal for chemical weathering of granite, sandstone and shale. A combination of the heat, and considerable water or rain in the rainy season allows the chemicals to attack the mineral grains especially the feldspars and convert them to clay. Saprolitic soils can form from such decomposition.