A pneumatolytic (see pneumatolysis) modification of a pre-existing igneous rock in which boron-rich, late-stage fluids react with the mineral assemblage of the primary rock (usually granite), eventually producing a rock consisting of tourmaline and quartz. Intermediate stages in the process, showing alkali feldspar partly replaced by rosettes of tourmaline, are commonly preserved in granites. Luxullianite, found at Luxulyan, Cornwall, is a fine example.