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

The crystal structure of walstromite, ideally BaCa2Si3O9, 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, = 69.638(2) , = 102.281(2) , = 96.855(2) , and V = 396.01(2) Å. The only previously published structure for walstromite was based on photographic film intensity data collected from synthetic BaCa2Si3O9 (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 CaSiO3-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 Ba by Ca are minimal and that walstromite is isomorphic with CaSiO3-walstromite, but topologically different from high-pressure wollastonite-II (Ca3Si3O9). 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.