The crystal structures have been refined for the first time for the strunzite-group minerals ferristrunzite and ferrostrunzite. Structure refinements are also presented for strunzite from three different localities. A common feature of all refined structural models is the presence of zeolitic water in the interlayer region, displaced by 0.4–0.5 Å from an inversion centre at (½ 0 ½). The refined occupancy of the site ranged from 44% to 100% in the samples studied. H-bonding associated with the zeolitic water causes large displacements, up to 0.7 Å, of neighbouring coordinated water molecules, relative to the structure with no zeolitic water. A new formula is proposed for strunzite-group minerals with divalent interlayer cations as M2+ Fe23+(PO4)2(OH)2·(6.5–x)H2O where M = Mn, Fe or Zn, and 0 < x < 0.5 accounts for varying degrees of dehydration. For ferristrunzite and other potential strunzite-group members with trivalent cations in the interlayer octahedron, the formula is M3+ Fe23+ (PO4)2(OH)3·(5.5–x)H2O.
