Prehnite interleaving biotite and graphite is reported from the Pre-Cambrian Kongsberg-Bamble Series. The mineral is regionally developed within plagioclase-biotite bearing rocks that have been metamorphosed under almandine-amphibolite and granulite facies conditions. The evidence suggests that hydrothermal sericitisation of the plagioclase feldspars has been responsible for the genesis of prehnite.