THE CRYSTAL CHEMISTRY OF POTASSIC-FERRISADANAGAITE

The crystal structure of potassic-ferrisadanagaite, (K0.66Na0.32)S0.98 (Ca1.73Na0.23Mn0.04)S2.00 (Mg0.55Fe2+2.16Mn0.25Fe3+1.11 Al0.78Ti0.19)S5.00 (Si5.33Al2.67)S8.00 O22 [(OH)1.21F0.41O0.38]S2.00, a 9.9309(1), b 18.0949(3), c 5.3681(1) Å, b 105.19(2) ° , V 930.9(1) Å3, C2/m, Z = 2, D(calc) = 3.36 g/cm3, has been refined using the Rietveld method to the following values: Rp = 3.5, RB = 2.6%. Mössbauer spectroscopy was used to characterize the Fe3+/(Fe2+ + Fe3+) value, and site populations were assigned on the basis of the refined site-scattering values at the A-, B-, and C-group sites, the unit formula calculated from the results of the chemical analysis, and the observed mean bond-lengths. The T(1) site is approximately half-occupied by [4]Al (2.15 apfu: atoms per for-mula unit) and the T(2) site is significantly occupied by [4]Al (0.66 apfu according to the <T(2)–O> distance). This distribution is in accord with no TAl–O–TAl linkages in the structure (in accord with the absence of Ca at the A site). In the presence of large amounts of TAl (>> 2 apfu), linkage between the octahedron strip and the double-chain of tetrahedra is maintained by incorpora-tion of large cations at the M(1), M(2) and M(3) sites (Fe2+! Mg, Fe3+! Al) and maximal kinking of the double-chain of tetrahedra (the latter being facilitated by K! Na at the A site)