Barringtonite, a new hydrous magnesium carbonate of composition MgCO3,2H2O, is recorded from Sempill Creek, Barrington Tops, New South Wales, where it occurs as nodular encrustations on the surface of olivine basalt. The mineral is triclinic, biaxial positive, has refractive indices α = 1·458, β = 1·473, γ = 1·501, 2Vγ = 73° 44′, and using Ito's method (1949) cell dimensions α = 9·155 Å, b = 6·202 Å, c = 6·092Å, α = 94· 00′, β = 95· 32′, and γ = 108° 72′. The three strongest lines on an X-ray powder photograph give d values of 8·682, 3·093, and 2·936 Å.