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1 Department of Earth System Sciences, Yonsei University, Seoul 120749, Korea and Physics Department, Brookhaven National Laboratory, Upton, New York 11973, U.S.A.
2 School of Chemistry, University of Birmingham, Birmingham, B15 2TT, U.K.
3 Geosciences Department, State University of New York, Stony Brook, New York 11794, U.S.A.
Correspondence: * E-mail: yongjaelee{at}yonsei.ac.kr
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 Na16–xCaxAl16+xSi24–xO80·24H2O, is removed from an aqueous environment and exposed to the atmosphere, it loses water and transforms to tetranatrolite, Na16–xCaxAl16+xSi24–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.
Key Words: Crystal structure • high pressure • phase transition • tetranatrolite • paranatrolite • synchrotron X-ray powder diffraction • pressure-induced hydration
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