Structuration under pressure: Spatial separation of inserted water during pressure-induced hydration in mesolite

In situ high-pressure single-crystal X-ray diffraction studies of mesolite, an aluminosilicate composed of stacks of Na⁺ -containing natrolite and Ca²⁺-containing scolecite layers in the ratio of 1:2, showed two discrete steps of pressure-induced hydration (PIH): first H₂O molecules are inserted into the natrolite layers between ∼0.5 and ∼1.5 GPa and subsequently into the scolecite layers. During the PIH in the natrolite layers, the coordination environment of Na⁺ changes from six to seven, the same as that of Ca²⁺ in the scolecite layers. While the natrolite layers behave as in the mineral natrolite, the scolecite layers show a different behavior from the mineral scolecite by adopting the super-hydrated natrolite-type structure at higher pressure, as a larger distortion is not favorable in the 1:2 layered framework. This spatial separation of inserted H₂O during PIH and the growing structural similarity of the two layers result in a weakening of k 3n reflections maintaining the 1:2 layer configuration. Our study of this unique behavior of mesolite provides a simple model of structuration under pressure, and the implications of our experimental findings are discussed.