Pressure-induced hydration in zeolite tetranatrolite

The tetranatrolite-paranatrolite transformation has remained a key problem in understanding the paragenesis of zeolites in the natrolite family. It is accepted that when paranatrolite, approximate formula Na_16-xCa_xAl_16+xSi_24-x O₈₀.24H₂O, is removed from an aqueous environment and exposed to the atmosphere, it loses water and transforms to tetranatrolite, Na_16-xCa_xAl_16+x Si_24-xO₈₀.nH₂O (n ≤ 24). Here we show that this transformation is not only reversible, but that tetranatrolite exhibits two sequential pressure-induced hydrations leading first to paranatrolite and then to a superhydrated tetranatrolite above 0.2 and 3.0 GPa, respectively. We have previously reported similar behavior for the corresponding system with an ordered Si/Al distribution, i.e., natrolite itself, however the ordered version of paranatrolite exists over a much smaller pressure range. The pressure-induced transformations of natrolite and tetranatrolite thus further supports the supposition that paranatrolite is a distinct mineral species, with a pressure-stability field dependent upon composition.