The crystal structure of gatehouseite

The crystal structure of the manganese phosphate mineral gatehouseite, ideally Mn ²⁺ ₅(PO ₄) ₂(OH) ₄, space group P2 ₁2 ₁2 ₁, a = 17.9733(18), b = 5.6916(11), c = 9.130(4) Å, V = 933.9(4) Å ³, Z = 4, has been solved by direct methods and refined from single-crystal X-ray diffraction data (T = 293 K) to an R index of 3.76%. Gatehouseite is isostructural with arsenoclasite and with synthetic Mn ²⁺ ₅(PO ₄) ₂(OH) ₄. The structure contains five octahedrally coordinated Mn sites, occupied by Mn plus very minor Mg with observed <Mn-O> distances from 2.163 to 2.239 Å. Two tetrahedrally coordinated P sites, occupied by P, Si and As, have <P-O> distances of 1.559 and 1.558 Å. The structure comprises two types of building unit. A strip of edge-sharing Mn(O,OH) ₆ octahedra, alternately one and two octahedra wide, extends along [010]. Chains of edge-and corner-shared Mn(O,OH) ₆ octahedra coupled by PO ₄ tetrahedra extend along [010]. By sharing octahedron and tetrahedron corners, these two units form a dense three-dimensional framework, which is further strengthened by weak hydrogen bonding. Chemical analyses by electron microprobe gave a unit formula of (Mn _4.99Mg _0.02) _Σ5.01(P _1.76Si _0.20As _0.07) Σ _2.03O ₈(OH) _3.97.