A new modification of slawsonite, SrAl2Si2O8 : its structure and phase transition

Slawsonite (SrAl 2 Si 2 O 8 ) from Sarusaka, Kochi Prefecture, Japan, is triclinic, space group P1, with a = 0.6145(1) nm, b = 0.8987(2) nm, c = 0.7456(3) nm, α = 90.01(3)°, β = 90.34(3)° and γ = 89.98(3)°, Z = 4 at T = 298 K. The cell dimensions are similar to the previous assignment to the monoclinic space group P2 1 /a. The average structure was refined to R = 0.049 from 2695 reflections. A model is proposed for the real structure taking into account about 200 superstructure reflections. Thermal analysis revealed a phase transition to P2 1 /a at 593 K which is considered a new type of transition for the paracelsian-slawsonite group.