Clinoptilolite is a sedimentary zeolite of extreme interest for industrial applications such as bulk separation and purification processes of waste waters and industrial effluents. It is considered a zeolite of the heulandite group with (Na+K) > Ca and Si/Al > 4. Only a few crystal structure refinements of very fine-grained (2-20 µm) natural sedimentary clinoptilolite are reported in the literature, the main reason being the presence of inseparable sample impurities such as clay minerals and feldspars intimately intergrown with clinoptilolite. The clinoptilolite studied in this work is of exceptional purity as the clinoptilolite phase fraction is around 99 wt% with trace amounts of quartz. This specimen allows for a direct structure refinement based on laboratory and synchrotron diffraction data. The sample was taken along the Rio Ayampe in the vicinity of the village of Guale and occurs as a series of white to greenish tuff layers situated in the pyroclastic Cayo Formation (Ecuador). This entirely zeolitised formation was deposited as the breakdown product of an intra-oceanic volcanic arc and constitutes a very large, recently discovered zeolite province accreted to the west Ecuadorian margin. To study the control of cation exchange on the crystal structure, the sample has been exchanged with K⁺, Na⁺ and Ca²⁺ by immersing equal portions in a nearly saturated solution of the corresponding acetate. The separate samples were stored in sealed containers in an oven at 60 °C to promote the exchange and the solutions were renewed periodically at 1, 4 and 10 days. By means of Atomic Absorption Spectrometry the cation concentrations in the out-coming solutions were analysed to evaluate the extent of exchange. The exchange was considered as completed after 20 days. X-Ray powder diffraction data of the natural and Na-exchanged forms were collected at a lab source and at the synchrotron Italian beamline BM08 at ESRF using an imaging plate system. The in-house data were collected with a Panalytical X'Pert Pro diffractometer with a θ/θ geometry, Cu Kα radiation, and a Real Time Multiple Strip (RTMS) detector. The data of the K- and Ca-exchanged forms were collected only using a conventional source. The Rietveld refinements were conducted with both GSAS and TOPAS packages.

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