Crystal chemistry of Rb-, Sr-, Ba-, Ca- and Pb-exchanged forms of natural hilairite

Cation-exchange properties of natural hilairite, Na 2 {ZrSi 3 O 9 }· 3H 2 O, 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 [Si 3 O 9 ] ∞ linked by isolated [ZrO 6 ] 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: Rb 1.80 Na 0.20 Zr[Si 3 O 9 ]·0.4H 2 O for Rb-exchanged hilairite ( R 3, a = 10.477(1), c = 15.377(2) A, R 1( F ) = 3.35% for 2771 reflections with F o > 4σ( F o )), Ba 0.96 H 0.08 Zr[Si 3 O 9 ]·3H 2 O ( R 3, a = 20.976(3), c = 7.857(2) A, R 1( F ) = 4.96% for 4921 reflections with F o > 4σ( F o )) and SrZr[Si 3 O 9 ]·1.5H 2 O ( R 3, a = 20.964(3), c = 7.836(2) A, R 1( F ) = 10.23% for 3598 reflections with F o > 4σ( F o )) for Ba- and Sr-exchanged hilairites, respectively, and Ca 0.83 H 0.34 Zr[Si 3 O 9 ]·3H 2 O ( R 3, a = 10.456(1), c = 7.995(2) A, R 1( F ) = 2.63% for 1477 reflections with F o > 4σ( F o )) and Pb 0.82 Na 0.18 H 0.18 Zr[Si 3 O 9 ]·2.9H 2 O ( R 3, a = 10.477(1), c = 7.994(2) A, R 1( F ) = 2.21% for 1165 reflections with F o > 4σ( F o )) 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.