Akrochordite, (Mn,Mg)5(OH)4(H2O)4(AsO4)2, monoclinic holosymmetric, a = 5.682(2), b = 17.627(5), c = 6.832(1) Å, β = 99.49(2)°, Z = 2, space group P21/c, Dobs = 3.19, 3.26, Dcalc = 3.26 g·cm−3 for (Mn0.80Mg0.20), has 12 non-H atoms in the asymmetric unit. R = 0.036 for 1295 independent Fo. Average distances are [6]Mn(1)–O 2.174,[6]Mn(2)–O 2.232,[6]Mn(3)–O 2.188, and [4]As–O 1.686 Å. Bond-distance averages suggest that Mg2+ substitutes for Mn2+, with a preference for the Mn(1) site followed by the Mn(3) site.

The perfect {010} cleavage results from a sheetlike structure with formula-unit composition [2M5(OH)4(H2O)4(AsO4)2] parallel to this plane. Of the six unique hydrogen bonds O–H· · ·O, four penetrate the c-axial glide planes at y = ¼, ¾ and bond symmetry-equivalent sheets together. Two of these bonds involve OW(2)→O(3) 2.79 and OH(6) → OW(2)3.02 Å, the arrows pointing to hydrogen-bond acceptors and the distances corresponding to oxygen-oxygen separations.

Upon projection along [012], it is seen that the sheets are made of fundamental building blocks based on [1M5ϕ2ϕ12] octahedral walls of amphibole type. In amphiboles, the ϕ′=(OH,F ), but in akrochordite it is apical O(1) of the [AsO4] tetrahedron. These walls are connected in stepwise fashion by O(2) and O(3) of [AsO4] to form the sheetlike modular units.

The remaining O(4) of [AsO4] receives three hydrogen bonds, two of which penetrate the c-axial glide planes. These have oxygen-oxygen separations OW(1)(2) → O(4) 2.59 and OW(2) → O(4) 2.62 Å. The remaining bond, within the modular unit, is OW(1) → O(4) 2.62 Å. Several examples of oxide terminal to a tetrahedral anionic group are known and include seamanite, metavauxite, and minyulite, all of which possess similar relatively short O–H· · ·O oxygen-oxygen distances, ranging from 2.58 to 2.80 Å.

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