THE CRYSTAL-STRUCTURE OF CHIAVENNITE

Chiavennite, CaMn(BeOH)(2)Si5O13.2H(2)O, is orthorhombic, space group Pnab, a = 8.729(5), b = 31.326(11), c = 4.903(2), Z = 4. The structure was determined by direct methods, using 1976 independent reflections, measured with an automated X-ray single crystal diffractometer. The a nisotropic least-squares refinement led to a final discrepancy index R = 0.041 for the 999 observed reflections with I greater than or equal to 5 sigma(I). Chiavennite has an interrupted framework of four-conne cted [SiO4] and three-connected [BeO4] tetrahedra, which consists of z ig-zag chains combined with a 4.8(2) 2D net. The framework density FD = 20.9 T/1000 Angstrom(3) classifies chiavennite as a zeolite. Nine-me mbered rings of tetrahedra form one-dimensional system of channels par allel to [001] with free diameters of 3.3 x 4.3 Angstrom. The extra-fr amework cations are Mn, in 6-fold coordination (octahedron, four O + t wo OH) and Ca, in g-fold coordination (square antiprism, four O + two OH + two H(2)0). Ca and H2O are located in the channels.