The crystal-chemistry of a series of natural phillipsites [ideal unit formula: K2(Na,Ca0.5)3[Al5Si11O32]· 12H2O, Z = 1, with a ~ 9.96, b ~ 14.20, c ~ 8.71 Å, and β ~ 24.97°, space group P21/m] from the Neapolitan Yellow Tuff was investigated by electron-microprobe analysis in the energy dispersive mode and powder X-ray diffraction using an in-house conventional diffractometer. A significantly different population of the extra-framework cations (i.e., K, Na and Ca) and water molecules was observed for crystals from different localities. All phillipsites from NYT can be described as “K-rich phillipsites”, due to the significantly high amount of potassium among the extra-framework cations. Rietveld structure refinements have been performed. The refined unit-cell parameters of the phillipsite samples show a little, but significant, control of the crystal-chemistry. The unit-cell volumes range between ~ 1008.7 and ~ 1011.6 Å 3. Samples with the highest amount of Na and the lowest amount of K show the largest unit-cell volume, whereas the smallest unit-cell volumes are observed for the samples with the lowest amount of Na and the highest amount of Ca. As to the unit-cell lengths, the longest c-axis with the highest distortion of the monoclinic unit-cell (i.e., with the highest β angle value) is observed for the samples with the lowest amount of Ca and the highest amount of K. The structure refinements performed in this study show a good general agreement with the complex configuration of the extra-framework population recently found on the basis of single-crystal data, with (1) one or two mutually exclusive K-rich sites, ~ 0–0.3 Å apart (i.e., K1 and K2); (2) one site partially occupied by Ca + Na; (3) seven independent sites occupied by water molecules (W1, W2, W3, W4, W4′, W5 and W6).

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