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 Ca (super 2+) -containing scolecite layers in the ratio of 1:2, showed two discrete steps of pressure-induced hydration (PIH): first H (sub 2) O molecules are inserted into the natrolite layers between approximately 0.5 and approximately 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 Ca (super 2+) 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 H (sub 2) O during PIH and the growing structural similarity of the two layers result in a weakening of k not = 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.