High-pressure behaviour of feldspathoids: the case of analcite

Feldspathoids are low silica minerals and, similar to zeolites, have large openings in the crystal structure. Elastic and structural behaviour of a natural cubic feldspatoid analcite (NaAlSi2O6•H2O) was investigated up to 8.5 GPa by in situ single-crystal X-ray diffraction. A first-order phase transition was observed at P = 0.98±0.07GPa. Lattice parameters and reflection conditions show that the HP-polymorph has a P -1 Sp. Gr.. Volume data of the low-P (cubic) and high-P (triclinic) polymorphs were fitted with a second- and third-order Birch-Murnaghan Equation of State [1], respectively. The refined elastic parameters are: V0= 2571.2(4)Å3, KT0= 56(3) GPa and K’= 4 (fixed), for the cubic polymorph, V0= 2613(10)Å3, KT0= 18(1) GPa and K’= 7.2(7), for the triclinic polymorph. The elastic behaviour of the HP-polymorph, calculated on the basis of the linearised bulk moduli, appears to be strongly anisotropic (K(a):K(b):K(c) = 2.07:1.36:1.00). Tetrahedral tilting produces the main deformation mechanism in response of the cubic-triclinic phase transition. The distortion of the secondary building units gives rise to a change of the 8- and 6-ring channels ellipticity. As a consequence, the extra-framework topological configuration changes: it appears in fact that the coordination number of part of the Na atoms becomes 7 (2H2O + 5 framework oxygens) instead of 6 (2H2O + 4 framework oxygens). [1]Birch,F., Phys. Rev., 1947, 71, 809.