The American Geological Institue (AGI) defines granites as an intrusive igneous rock (cooled slowly in the deep upper part of the Eart’s crust) composed of 25% to 35% quartz and over 50% potassium- and sodium-rich feldspars, with a common accessory mineral of less than 20%, usually muscovite (clear mica), biotite (dark, iron-bearing mica), or hornblende (amphibole).
