Crystal structure of the hydrated sulphate pickeringite (MgAl2(SO4)(4) center dot 22H(2)O): X-ray powder diffraction study

The crystal structure of pickeringite (ideal formula MgAl2(SO4)(4). 22H(2)O ) from Roccalumera (Messina, Italy), a hydrated sulphate of the halotrichit e group, has been determined by X-ray powder diffraction with the Rietveld method. The space group has been confirmed to be P2(1)/c with unit cell par ameters: a = 6,1844(2) Angstrom, b = 24.2715(9) Angstrom, c = 21.2265(7) An gstrom and beta = 100.326(4)degrees. The comparison of these powder data wi th those reported in the PDF-2 database (12-299) evidences strong differenc es in both the cell parameters and Bragg reflection positions, suggesting t he redefinition of the PDF card for this mineral. The results of the Rietve ld refinement confirm the close correspondence between the structures of pi ckeringite and apjohnite (MnAl2(SO4)(4).22H(2)O).