Summary: Fenitized quartzites are described from three localities around the Borralan alkaline igneous complex. Chemical and electron-probe analyses of amphiboles, pyroxenes, micas, and feldspars are given. Early-formed amphiboles are calcic, but these are replaced by alkaline varieties with increasing fenitization. Rock analyses indicate a ‘straight line’ rate of increase of Ti, Fe²⁺ + Fe³⁺, Mg, Al, K, and a more variable increase of Ca and Na atoms, at the expense of Si atoms. The simple composition of the unaltered quartzite allows the accurate calculation of bulk changes during fenitization. The final product of the alkali metasomatism is a syenite lying close to the albite-orthoclase minimum though the primary source of the fenitizing fluids was probably a highly potassic magma.