Elastic and structural behaviour of analcite at high pressure

Analcite (ANL), NaAlSi2O6H2O, is a natural zeolite (or felspathoid). The crystal structure is based on a Secondary Building Unit (SBU) consisting of four-member tetrahedral rings, which are joined to form a complex tetrahedral framework with two different systems of channels: highly distorted 8-rings and regular 6-ring channels. Na and H2O lie into the structural voids. Two H2O and four framework oxygens coordinate the Na-atoms. Elastic and structural behaviour of a natural cubic ANL from Sardinia (Italy) was investigated at HP by in situ single-crystal X-ray diffraction. A first-order phase transition was observed at P = 0.98(7)GPa. Lattice parameters and reflection conditions confirm that the HP-polymorph has a P-1 space group. The experiment is in progress but no further phase-transition has been observed at least up to 6.3 GPa. Fitting the volume data of the cubic polymorph with a BM-EoS we obtain: V0= 2571.2(4)Å3, KT0= 56(3) GPa and K’= 4 (fixed). For the triclinic polymorph: V0= 2618(13)Å3, KT0= 17(1) GPa and K’= 7(1). Tetrahedral tilting produces the relevant structural variations in response of the cubictriclinic phase transition. The SBU distortion gives rise to a change of the 8- and 6-ring channels ellipticity and of the extra-framework content topological configuration: it appear in fact that the coordination number of half of the Na atoms is 7 (2H2O + 5 framework oxygens) instead of 6.