Rigid-body character of the SO4 groups in celestine, anglesite and barite

The crystal structures of natural celestine (Sr-1.00)SO4, anglesite (Pb0.99 Sr0.01)SO4, and barite (Ba0.99Sr0.01)SO4 have been refined in space group P bnm utilizing rotating anode, Mo X-ray diffraction data from single crystal s. Unit-cell parameters for celestine are a 6.8671(7), b 8.3545(8), and c 5 .3458(6) Angstrom, for anglesite, a 6.9549(9), b 8.472(1), and c 5.3973(8) Angstrom, and for barite, a 7.154(1), b 8.879(2), and c 5.454(1) Angstrom. Structural data are presented for these sulfates with greatly improved prec ision over previous studies owing to high peak-to-background intensity rati os and precise analytical absorption corrections. The final model R(F) valu es are 0.025, 0.041, and 0.019, for celestine, anglesite, and barite, respe ctively. The average bond-distance from divalent cation to the nearest twel ve oxygen atoms is 2.827(1) Angstrom in celestine, 2.864(5) Angstrom in ang lesite, and 2.951(2) Angstrom in barite. The average sulfur-to-oxygen bond distance is 1.475(2) Angstrom in celestine, 1.476(6) Angstrom in anglesite, and 1.476(2) Angstrom in barite. The sulfate tetrahedra in each structure show very similar distortions that are attributed to the bonding of the var ious oxygen atoms to the divalent cations, which is similar in each structu re. Thus, the different metal cations do not seem to affect the size or sha pe of the sulfate tetrahedra. An analysis of the displacement parameters su ggests that the SO4 groups behave as rigid molecular units, with an apparen t shortening of the S-O bonds of 0.008-0.010 Angstrom.