Optical and physical properties of zeolite minerals are summarized in Table 3.4 through 3.36. These properties have been selected as representative properties. In some cases, it is difficult to assingn a particular x-ray powder diffraction pattern to a certain species because variations do occur in the x-ray pattern as a result of variation in the exchange cation composition. The table for each mineral in cludes x-ray powder data which have been obtained from selected sources including the Powder Diffraction File. The intensities of the invididual x-ray reflections are those given in the source. In some instances the intensities are given on a relative scale, such as vs, s, ms, m, w, vw, and vvw, corresponding to very strong, strong, medium strong, medium, weak, very weak and very very weak. For the identification of zeolites which occur as fine-grained crystals in sedimentary rocks, x-ray diffraction is the primary tool. Routine diffractometer analysis is generally sufficient for identification of zeolites which occur in rock in concentrations of more than 10%. In order to identify zeolites which are present in less than this amount, some method of concentration is necessary. Although special optical data may be reasonably sufficient to identify a larger zeolite crystal of the igneous zeolites, it generally is a supplement to the modern x-ray method. The refractive index, for example, has been used for distinguishing clinoptilolite from heulandites.