Abstract: A low-charge corrensite, i.e., a regular 1:1 interstratified smectite/chlorite was found in veins and fissures of an extensively fractured dolomite south of Perth, Ontario, Canada. Total chemical analysis indicated that the mineral was trioctahedral, having a structural formula corresponding to: ( Ca 0.17 ) ( Mg 7.36 Al 0.90 Fe 3+ F 0.45 e 2+ ) 0.42 ( Si 6.06 Al 1.94 ) VI IV O 20 (OH) 10 . The coefficient of variation (CV) of the d(00l) spacings calculated for the mineral from X-ray powder diffraction data was 0.42, well below the maximum recommended value for corrensite, 0.75. The corrensite coexists with phlogopite, which is present as small (∼0.5 mm) crystals throughout the rock and locally as large (∼ 5 cm) crystals in pegmatitic veins. Microscopic observations of large phlogopite crystals partially altered to corrensite suggest that corrensite formed at the expense of phlogopite by hydrothermal alteration.