Since its first discovery in nature, natrolite has been largely known as a sodium aluminosilicate zeolite, showing very limited preference toward cation exchange. Here we show that fully K-exchanged natrolite can be prepared from natural Na-natrolite under mild aqueous conditions and used to subsequently produce Rb-and Cs-exchanged natrolites. These cation-exchanged natrolites exhibit successive volume expansions by ca. 10, 15.7, and 18.5% for K-, Rb-, and Cs-forms, respectively, compared to the original Na-natrolite. This constitutes the largest, ever-reported volume expansion via cation substitution observed in zeolites and occurs by converting the elliptical channels into progressively circular ones. The observed cation-dependent changes in the channel volume and shape thus show the flexibility limits of the natrolite framework and suggest the possible existence of compositionally altered analogues in suitable environments as well as a novel means to tailor the cation selectivity of this class of small pore zeolites toward various industrial and environmental applications.

High-pressure synchrotron X-ray powder diffraction studies of a series of alkali-metal-exchanged natrolites, A16Al16Si24O80·nH2O (A=Li, K, Na, Rb, and Cs and n=14, 16, 22, 24, 32), in the presence of water, reveal structural changes that far exceed what can be achieved by varying temperature and chemical composition. The degree of volume expansion caused by pressure-induced hydration (PIH) is inversely proportional to the non-framework cation radius. The expansion of the unit-cell volume through PIH is as large as 20.6% in Li-natrolite at 1.0 GPa and decreases to 6.7, 3.8, and 0.3% in Na-, K-, and Rb-natrolites, respectively. On the other hand, the onset pressure of PIH appears to increase with non-framework cation radius up to 2.0 GPa in Rb-natrolite. In Cs-natrolite, no PIH is observed but a new phase forms at 0.3 GPa with a 4.8% contracted unit cell and different cation-water configuration in the pores. In K-natrolite, the elliptical channel undergoes a unique overturn upon the formation of super-hydrated natrolite K16Al16Si24O80·32H2O at 1.0 GPa, a species that reverts back above 2.5 GPa as the potassium ions interchange their locations with those of water and migrate from the hinge to the center of the pores. Super-hydrated zeolites are new materials that offer numerous opportunities to expand and modify known chemical and physical properties by reversibly changing the composition and structure using pressure in the presence of water.

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Pressure drastically alters the chemical and physical properties of materials and allows structural phase transitions and chemical reactions to occur that defy much of our understanding gained under ambient conditions. Particularly exciting is the high-pressure chemistry of xenon, which is known to react with hydrogen and ice at high pressures and form stable compounds. Here, we show that Ag16Al16Si24O8·16H2O (Ag-natrolite) irreversibly inserts xenon into its micropores at 1.7 GPa and 250 °C, while Ag(+) is reduced to metallic Ag and possibly oxidized to Ag(2+). In contrast to krypton, xenon is retained within the pores of this zeolite after pressure release and requires heat to desorb. This irreversible insertion and trapping of xenon in Ag-natrolite under moderate conditions sheds new light on chemical reactions that could account for the xenon deficiency relative to argon observed in terrestrial and Martian atmospheres.

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Water is the most abundant volatile component in the Earth and influences both physical and19 chemical properties of the Earth materials. It continuously enters the Earth through subduction 20 zones where it reduces the melting temperature of rocks to generate magmas while the rest travels 21 further deep into the Earth. Our understanding of the water cycle in the Earth has emphasized 22 dehydration processes along the subduction zones. Here we show that the formation and subsequent 23 breakdown of super-hydrated kaolinite have important implications for water transport, volcanism, 24 and possibly seismicity along the subduction zones. We measured in-situ and time-resolved high-25 pressure/high-temperature synchrotron X-ray diffraction and infrared spectra to characterize 26 2 structural and chemical changes of kaolinite at conditions corresponding to those found in subduction 27 zones. Synchrotron X-ray powder diffraction patterns of kaolinite at 2.7(1) GPa after heating to 28 200 °C in the presence of water, a condition corresponding to a depth of about 75km in cold slabs, 29 show the appearance of a reflection with a d-spacing near 10Å which arises from pressure-induced 30 insertion of water. This new super-hydrated phase of kaolinite has a ~31 % larger unit cell volume 31 and a ~ 8.4% lower density than the original kaolinite and has, with 29 weight-% H 2 O, the highest 32 water content of any known aluminosilicate mineral in the Earth. As pressure and temperature 33 approach 19 GPa and ca. 800 ˚C, we observe the sequential breakdown of super-hydrated kaolinite to 34 phase-Pi, diaspore, and topaz-OH along with the formations of coesite and stishovite. Breakdown of 35 super-hydrated kaolinite in cold slabs subducted below 200 km then leads to the release of water that 36 may further affect seismicity and help fuel arc volcanism at the surface. 37 38 Current predictions of the global H 2 O flux into the deep mantle amounts to about one ocean mass over 39 the age of the Earth 1 . Present-day estimates for the subduction efficiency reveal that ca. 68% of subducted 40 water outgasses through arc volcanism 1-5 . Trapped in the oceanic sediments or oceanic crust in hydrous 41 minerals 6 containing H 2 O molecules or OHgroups such as serpentine 7 , lawsonite 8,9 , phlogopite 10 , and 42 amphibole 11 , 68% of the subducted water is released at different depths along the subduction zone: 43 amphiboles at 75 km, phlogopite at 200 km and lawsonite at 300 km 12 . Hyndman et al. (1997) 44 demonstrated that clay sediments at the slab surface control the updip limit of a seismogenic zone in 45 subduction thrust faults 13 . Interestingly, the temperature ranges of phase transformation from smectite to 46 illite/cholorite coincide with this updip seismogenic limit. At depths between 410 and 660 km in the 47 Earth's mantle transition zone, the presence of water-containing minerals such as wadsleyite and 48 ringwoodite induce further dehydration melting 14-16 . Dehydration embrittlement where released aqueous 49 fluids induce br...

When the small-pore zeolite natrolite is compressed at ca. 1.5 GPa and heated to ca. 110 °C in the presence of CO(2), the unit cell volume of natrolite expands by 6.8% and ca. 12 wt % of CO(2) is contained in the expanded elliptical channels. This CO(2) insertion into natrolite is found to be reversible upon pressure release.

High-pressure ion exchange of small-pore zeolite K-natrolite allows immobilization of nominally non-exchangeable aliovalent cations such as trivalent europium. A sample exchanged at 3.0(1) GPa and 250 °C contains about 4.7 Eu(III) ions per unit cell, which is equivalent to over 90 % of the K(+) cations being exchanged.