overite, [Mg(H2O4]2[Ca2Al2(OH)2(PO4)4], Z = 4, a = 14.723(14), b = 18.746(16), c = 7.107(4) A, orthorhombic, space group Pbca; and segelerite, [Mg(H2O)4]2

[Ca2Fe23+(OH)2(PO4)4]
⁠, Z = 4, a = 14.826(5), b = 18.751(4), c = 7.307(1) A, orthorhombic, space group Pbca, are related in a combinatorial way to jahnsite, [Mg(H2O)4]2
[CaMn2+Fe23+(OH)2(PO4)4]
⁠, Z = 2, a = 14.94(2), b = 7.14(1), c = 9.93(1) A, β = 110.16(8)°, monoclinic, space group P2/a. R = 0.090 for 19l5 reffections (overite) and 0.236 for 2387 reflections (segelerite).

Overite, segelerite, and jahnsite are based on dense slabs of composition

[X22+M23+(OH)2(PO4)4]
whose maximal symmetry is Bmam. Linkages to the slabs by [Mg(Op)2(H2O)4] octahedra determine the discrete kinds of isomerism. In overite and segelerite, the arrangement is trans-[Mg(OP)2(H2O)4], and in jahnsite it is alternately cis-[Me(OP)2(H2O)4] and trans-[Mg(OP)2(H2O)4], where OP is a phosphate oxygen.

Overite, which is well-refined, has average distances (6)Ca-O 2.40, (6)Al-O 1.91, (6)Mg-O 2.08, (4)P(1)-O 1.54, and (4)P(2)-O 1.54A. A herring-bone pattern of hydrogen bonds is arranged normal to the [010] direction.

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