Structure of walstromite, BaCa₂Si₃O₉, and its relationship to CaSiO₃-walstromite and wollastonite-II

The crystal structure of walstromite, ideally BaCa₂Si ₃O₉, was refined with data from single-crystal X-ray diffraction on a natural specimen from the type locality Esquire No. 8 claim, Big Creek, Fresno County, California, U.S.A. It is triclinic, with space group P1 and unit-cell parameters a = 6.7335(2), b = 9.6142(3), c = 6.6859(2) Å, a = 69.638(2)°, β = 102.281(2)°, ? = 96.855(2)°, and V = 396.01(2) Å3. The only previously published structure for walstromite was based on photographic film intensity data collected from synthetic BaCa ₂Si₃O₉ (Dent Glasser and Glasser 1968). Due to uncertainty in oxygen positions, the reported final R-factor was 0.16. The current refinement yielded an R-factor of 0.030 with the inclusion of anisotropic displacement parameters. Walstromite is a Ba-Ca cyclosilicate characterized by Si₃O₉ three-membered rings. It is related to the important calcium silicate group of minerals, especially to CaSiO ₃-walstromite, through the substitution of Ba into one of the three distinct Ca sites. Joswig et al. (2003) suggested that the structural changes caused by the replacement of Ba²⁺ by Ca²⁺ are minimal and that walstromite is isomorphic with CaSiO₃-walstromite, but topologically different from high-pressure wollastonite-II (Ca ₃Si₃O₉). Our study demonstrates that wollastonite-II and CaSiO₃-walstromite are identical phases, and are isostructural with walstromite. This isomorphism implies that the high-pressure CaSiO₃ phase may be a potential host for large cations in deep Earth environments.