Acid rain will dissolve the rock. The feldspars break down to clays and the dark, iron bearing minerals break down to iron oxides or hydroxides like goethite or nontronite. quartz is more resistant so it collects as residual grains.
What is Hornblende? Hornblende is a field and classroom name used for a group of dark-colored amphibole minerals found in many types of igneous and metamorphic rocks. These minerals vary in chemical composition but are all double-chain inosilicates with very similar physical properties.