(Na, K)-exchanged forms of laumontite, co-called “primary leonhardite”, can be prepared under hydrothermal conditions. The products show two steps of dehydration at about 180 and 240°C on TG-DTA curves, the same as previous reports about primary leonhardite. XRD data recorded under controlled relative-humidity suggest that the synthetic “primary leonhardite” maintains a similar framework structure to the fully hydrated phase of laumontite between 0 and 100% RH at 25°C. The number of water molecules varied from 13 to 17 per unit cell when replacing Ca by alkaline cations. The alkaline extra-framework cation sites of “primary leonhardite” are determined by Rietveld's method.
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