Wickenburgite, a double-layer silicate Pb₃Al^[6][Ca^[6]Al^[4]Si^[4]₁₀O₂₇(H₂O)₃]·H₂O: Crystal chemistry and thermal behaviour

Abstract

To clarify discrepancies in the literature with regard to the water content and crystal structure of wickenburgite, chemical and thermal analyses and IR spectroscopic measurements were made and the structure was redetermined with single-crystal X-ray diffraction data. It was found that the idealised composition of the mineral is Pb₃Al₂CaSi₁₀O₂₇·4 H₂O rather than Pb₃Al₂CaSi₁₀O₂₇·3 H₂O as reported by Lam et al. in 1994. Based on chemical analysis and crystal structure determination, there is considerable isomorphous replacement on the cation positions, leading to the crystal-chemical formula

Pb₃(Al_0.79Pb_0.22)_Σ1.01^[6]{uB, 4, 2_∞²} [Ca_1.02^[6](Al_0.81Fe_0.06Si_0.14)_Σ1.01^[4]Si_10.00^[4]O₂₇(H₂O)₃]·H₂O.

With regard to crystal structure, that described by Lam et al. is found to be essentially correct, except for an additional H₂O molecule per formula unit, whereas the structure suggested by Penna & Giese in 1968 and by Hesse & Liebau in 1992 and 1993 is incorrect. The wickenburgite structure contains silicate double-layers in which one out of twelve [(Al, Si)O₄] tetrahedra is replaced by an [O₃Ca(H₂O)₃] octahedron. Adjacent double layers are held together by Al³⁺ and Pb²⁺ cations that are octahedrally and irregularly eight-fold coordinated, respectively. The fourth H₂O molecule per formula unit occupies a small cavity which is formed by six oxygen atoms from two adjacent double layers and three interlayer lead ions, thus shielding the repulsive forces between the lone-pair electrons of the latter. Minor but varying amounts of adhering, non-crystalline water is reversibly lost upon heating below 100°C. The cavity water is steadily and irreversibly lost between approximately 100 and 550°C (at a heating rate of 10°C per minute). Before all cavity water has been driven out, water coordinating the Ca²⁺ ions begins to escape. This dehydration process is accompanied by an approximately 4.4 percent reduction of the unit-cell volume without substantial disruption of the structure. At 740 °C the dehydration is not yet complete. The structure of wickenburgite is related to that of jagoite which contains both silicate single- and double-layers. In the double-layers of wickenburgite one out of twelve [(Al, Si)O₄] tetrahedra is replaced by an [O₃Ca(H₂O)₃] octahedron whereas in jagoite within the double-layers two out of twelve [(Fe, Si)O₄] tetrahedra are replaced by [PbO₃] pyramids.