Structure evolution of hydroxyapophyllite-(K) under high pressure

Abstract

Abstract The high-pressure structural evolution of a natural hydroxyapophyllite-(K) K0.96 Ca4.01[Al0.01Si7.99O20]((OH)0.95F0.05)·(H2O)8.14, Z = 2, a = 8.9699(1), c = 15.8934(3) Å, space group P4/mnc, from the Hatrurim Basin, Negev Desert, compressed in penetrating (ethanol:water 8:1 mixture) medium up to 5 GPa, was studied by single-crystal X-ray diffraction with a diamond anvil cell. The results clearly demonstrate the absence of pressure-induced hydration in the structure. Within 3 GPa the compression mechanism is similar to that known in fluorapophyllite-(K). The compression in the plane of silicate layer proceeds via the relative rotation of the 4-membered rings. The compression along the c-axis proceeds through the shortening of the interlayer distance, whereas the thickness of silicate layer remains almost unchanged. As a result, the pressure-induced changes in the unit cell metrics are similar to those for fluorapophyllite-(K). At about 3 GPa hydroxyapophyllite-(K) undergoes a phase transition with the symmetry lowering to orthorhombic (space group Pnnm). The symmetry of the high-pressure phase allows deformation of the four-membered rings of the silicate layer, which is impossible within tetragonal symmetry. In this case, the structure is compressed much more along the a-axis than along the b-axis. As a result, the orthorhombic phase of hydroxyapophyllite-(K) is more compressible compared to fluorapophyllite-(K).