The proplem of assingning a specific set of x-ray powder data to a zeolite is exemplified in Fig. 3.9. The x-ray powder patterns are shown for three phillipsite specimens from three different localities. Two of these are in igneous rock and the third is a deep-sea specimen from the Sylvania sea mount. Also included are the x-ray powder patterns for structurally related zeolites, harmotome and gismondine. It appears superficially that harmotome is no less similar to phillipsite than is gismondine, although gismondine has a different framework structure.