Summary: 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+ (Mg5·84Mn0·01Al0·03Fe0·08) (Si6·99Al1·01)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.