Rhodesite from Zeilberg, Unterfranken, Germany (Z), from Trinity County, California, USA (TC), and from San Venanzo, Rieti, Italy (SV), is orthorhombic with space group Pmam. Crystallographic data, given in the order (Z), (TC), (SV), are: a0 = 23.416(5)/23.444(5)/23.428(20) angstrom, b0 = 6.555(1)/6.553(1)/6.557(8) angstrom, c0 = 7.050(1)/7.055(1)/7.064(8) angstrom, Z = 2, D(x) = 2.268(1)/2.264(1)/2.261(4) g . cm-3 calculated for HKCa2Si8O19 . 5H2O. The crystal structure was refined to R(unweighted) = 0.044/0.079/0.044 and R(weighted) = 0.036/0.055/0.045 using 939/898/536 non-equivalent reflections. The structure contains loop-branched dreier double layers of corner-sharing [SiO4] tetrahedra. Channel-like pores running parallel to [010] and [001] between the two sublayers of a double layer are occupied by potassium ions and water molecules. Adjacent double layers are held together by rather strong hydrogen bonds (ca. 2.60 angstrom) and by two sets of calcium ions which are octahedrally coordinated by six terminal oxygen atoms from the silicate layers, and by four terminal oxygen atoms plus two water molecules, respectively. Although no sodium has been found during refinement of the site occupation factors it is concluded from comparison with the closely related crystal structures of delhayelite, hydrodelhayelite, macdonaldite and monteregianite that stoichiometric rhodesite is the sodium-free endmember of a solid solution HK1-xNa(x+2y)Ca2-y{lB,3,2 infinity 2}[Si8O19] . (6 - z) H2O. Conditions for the formation of double-layer silicates of the rhodesite family in nature and for their synthesis are discussed.
