Gatta, G. Diego; Cappelletti, Piergiulio; Langella, Alessio
Crystal-chemistry of phillipsites from the Neapolitan Yellow Tuff
European Journal of Mineralogy Volume 22, 6 (2010), p. 779 - 786
The crystal-chemistry of a series of natural phillipsites [ideal unit formula: K2(Na,Ca0.5)3[Al5Si11O32]·12H2O,Z= 1, witha∼ 9.96,b∼ 14.20,c∼ 8.71 Å, andβ∼ 124.97°, space groupP21/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 Å3. 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 longestc-axis with the highest distortion of the monoclinic unit-cell (i.e., with the highestβangle 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).