Powder Rietveld refinement of armstrongite, CaZr[Si6O15] center dot 3 H2O

Crystal data and results of structure refinement using powder X-ray diffrac tion and Rietveld analysis for armstrongite, CaZr[Si6O15]. 3H(2)O, are repo rted. Experimental data were collected using MoKalpha1 radiation over the r ange 2 degrees - 51.98 degrees 2 theta. Armstrongite is monoclinic (space g roup C2, Z = 4), with a = 14.018(1) Angstrom, b = 14.133(1) Angstrom, c = 7 .840(1) Angstrom, beta = 109.40(1)degrees. The crystal structure, reported by Kashaev and Sapozhnikov (1978) has been refined by Rietveld analysis, re sulting in R-wp = 2.75. The structure is based on a heterogeneous octahedra -tetrahedra framework (ZrSi6O15) built by corrugated silicate sheets connec ted by ZrO6 octahedra. The seven-fold Ca polyhedron contains 2 H2O molecule s. Two new sites fur H2O molecules on the 2-fold axis in the voids of the z eolite like framework allowed to achieve the complete correspondence betwee n chemical and structural data.