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 . Chains of edge-and corner-shared Mn(O,OH) 6 octahedra coupled by PO 4 tetrahedra extend along . 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.