This saponite, which is closely associated with zeolites such as stillbirth, heulandite, thomsonite, and gonnardite, occurs in vesicles in basalt. It has the composition: 1·02M⁺ (Mg_5·84Mn_0·01Al_0·03Fe_0·08) (Si_6·99Al_1·01)O₂₀(OH)₄, according to a new method for calculating the ionic formulae of smectites: this method also enables determination of the temperature at which all sorbed moisture is removed—in this instance about 800°C. The exchangeable cations are largely Ca²⁺. The chemical, thermal, and X-ray data are discussed with reference to other saponite samples. Electron-microscope examination showed a tendency to elongation in the direction of the α-axis. It is concluded that the saponite is of excellent purity and free from interstratification.