Cation-exchange properties of natural hilairite, Na2{ZrSi3O9}· 3H2O, in aqueous salt solutions at 150 °C and the crystal structures of its Rb-, Ca-, Sr-, Ba- and Pb-exchanged forms have been studied on single crystals. All studied samples retain the mixed framework of “parent” hilairite consisting of helical chains [Si3O9] linked by isolated [ZrO6] octahedra. The main differences between the crystal structures of these cation-exchanged forms of hilairite and other representatives of the hilairite group are connected with the number and positions of extra-framework cations and water molecules, framework distortion, unit-cell dimensions and space-group symmetry. The single-crystal structure refinements (X-ray diffraction data) gave the following structural formulas for the studied samples: Rb1.80Na0.20Zr[Si3O9]·0.4H2O for Rb-exchanged hilairite (R3, a = 10.477(1), c = 15.377(2) Å, R1(F) = 3.35% for 2771 reflections with Fo > 4σ(Fo)), Ba0.96H0.08Zr[Si3O9]·3H2O (R3, a = 20.976(3), c = 7.857(2) Å, R1(F) = 4.96% for 4921 reflections with Fo > 4σ(Fo)) and SrZr[Si3O9]·1.5H2O (R3, a = 20.964(3), c = 7.836(2) Å, R1(F) = 10.23% for 3598 reflections with Fo > 4σ(Fo)) for Ba- and Sr-exchanged hilairites, respectively, and Ca0.83H0.34Zr[Si3O9]·3H2O (R3, a = 10.456(1), c = 7.995(2) Å, R1(F) = 2.63% for 1477 reflections with Fo > 4σ(Fo)) and Pb0.82Na0.18H0.18Zr[Si3O9]·2.9H2O (R3, a = 10.477(1), c = 7.994(2) Å, R1(F) = 2.21% for 1165 reflections with Fo > 4σ(Fo)) for Ca- and Pb-exchanged hilairites, respectively. All samples were twinned by merohedry and, except the Ca-exchanged sample, also racemically. The results of this study strongly indicate that natural cation-exchange processes can affect hilairite-group minerals under late-hydrothermal conditions.

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