An altered, light-coloured, igneous rock consisting of white mica and quartz. The rock forms by the reaction of crystalline granite with hot fluorine-rich vapour derived from the crystallizing granite at a deeper level in the intrusion. The granite mineralogy is unstable in the presence of this vapour and reacts to give the stable mica-quartz assemblage. This process is termed pneumatolysis. Greisens are found as marginal modifications to granite adjacent to mineral veins, as thin veins and dykes in granite fissures, and as large bodies at the tops and sides of granite intrusions.