The crystal structure of pickeringite (ideal formula MgAl2(SO4)4. 22H2O) from Roccalumera (Messina, Italy), a hydrated sulphate of the halotrichite group, has been determined by X-ray powder diffraction with the Rietveld method. The space group has been confirmed to be P21/c with unit cell parameters: a = 6.1844(2) Å, b = 24.2715(9) Å, c = 21.2265(7) Å and β = 100.326(4)°. The comparison of these powder data with those reported in the PDF-2 database (12-299) evidences strong differences in both the cell parameters and Bragg reflection positions, suggesting the redefinition of the PDF card for this mineral. The results of the Rietveld refinement confirm the close correspondence between the structures of pickeringite and apjohnite (MnAl2(SO4)4·22H2O).