The structure of slawsonite, a strontium analog of paracelsian

Slawsonite is a new mineral from the Triassic Martin Bridge Formation in Wallowa County, Oregon. Its composition (Sr_0.87Ca_0.13Al₂Si₂O₈), cell dimensions (a = 8.888, b = 9.344, c = 8.326 Å, ʲ = 90.33°), and P2₁/a space group suggest it is isostructural with paracelsian, BaAl₂Si₂O₈. A structure refinement using 1617 twice-measured reflections with $|Fobs|>3σ^$ was undertaken by conventional methods to R = 0.048. Like paracelsian, slawsonite has a completely ordered Al/Si distribution in its infinite 3-D framework of corner-sharing tetra-hedra. The mean Al–O distances at T10 and T2m are both 1.748 Å; mean Si-O distances are 1.623 and 1.624 Å for the T1m and T20, respectively. The former are equal to those in paracelsian, while the latter are 0.004 Å longer on the average. The Sr atom is 7-coordinated; (Sr–O) = 2.630 Å. There is a high correlation among individual T–0 distances, the mean intertetrahedral angle T–O–T, and the mean of the three O–T–O angles involved in the T–O bond.