Pollucite, the only caesium silicate mineral known, is recorded from a new locality. It has a refractive index of 1.5173 (Na light) and the dispersion nF—nc= .0083. A chemical analysis of the mineral is given and from a consideration of this and other available analyses, it appears that an isomorphous series exists between pollucite and analcite. This series results from the isomorphous replacement of Cs+ by Na+ with the concomitant introduction of a water molecule into the unit cell. The variation of the refractive index within this series is shown graphically. Some dehydration results obtained on the pollucite from Karibib are briefly discussed.