Description and unique crystal-structure of waterhouseite, a new hydroxy manganese phosphate species from the Iron Monarch deposit, Middleback Ranges, South Australia

Waterhouseite from the Iron Monarch mine, Iron Knob, South Australia, is a new hydroxy manganese phosphate species that has a unique crystal-structure. The mineral was found in a carbonate-rich zone with gatehouseite, seamanite, rhodochrosite, shigaite, barite, hausmannite and hematite. It occurs as divergent sprays of orange-brown to dark brown bladed crystals up to 1 mm in length but only up to 20 μm in thickness. The crystals are transparent with a pearly luster on cleavages, but it is vitreous to pearly on the tabular faces. The mineral is brittle, with a conchoidal fracture and a yellowish brown streak. There is a perfect cleavage on (100) and a probable cleavage on (001). The crystals show the principal forms {100} (dominant), {010}, {011} and {001}. All crystals are twinned on (100) by non-merohedry. The Mohs hardness is estimated to be ∼4, and the measured density is 3.55(5) g/cm³ (calculated density is 3.591 g/cm³). Crystals are biaxial negative and length-slow, with α 1.730(3), β ∼1.738 and γ 1.738(4), but 2V could not be measured. Interference colors are normal, implying the absence of optical dispersion. The optical orientation is XYZ = bac (pseudo-orthorhombic), and the pleochroism is X pale brownish, Y brown-yellow, Z pale brownish, with absorption Z = X > Y. Electron-microprobe analyses yielded the empirical formula Mn_7.29[(P_1.81As_0.07 V_0.04)_∑1.92O_7.68](OH,O) _8.32, calculated on the basis of 16 O atoms. The simplified formula is Mn₇(PO₄)₂(OH)₈, in agreement with the crystal-structure determination. The strongest five lines in the powder X-ray-diffraction pattern [d in Å(I)(hkl)] are: 4.436(70)(111), 3.621(100)(202), 3.069 (50)(311), 2.941(40)(013), and 2.780(35)(020). Unit-cell parameters refined from powder-diffraction data, a 11.364(6), b 5.570(2), c 10.455(3) Å, β 96.61(3)°, V 657.4(2) Å ³ (Z = 2), agree very well with those refined from the single-crystal data. The crystal structure was solved by direct methods and refined in space group P2₁/c to R1(F) = 5.15% and wR2_all(F²) = 16.28% using data from a twinned crystal (by non-merohedry) with 1400 "observed" reflections with F_o. > 4o-(F_o). The crystal structure is characterized by a dense, complex framework of Mn(O,OH)₆ octahedra and PO₄ tetrahedra, which are linked by both edges and comers. Two different subunits can be recognized in the structure: arsenoclasite-type strips of edge-sharing octahedra (fragments of brucite-pyrochroite-type sheets of octahedra) and finite chains of edge-sharing octahedra (fragments of infinite rutile-type chains). The PO₄ tetrahedra provide a connection between the strips and the chains. Single-crystal Raman spectra confirm weak hydrogen bonding. A unique feature of the structure is that the single PO₄ tetrahedron shares two of its edges with Mn(O,OH)₆ octahedra. Only two synthetic anhydrous metal arsenates are known that show a corresponding sharing of two edges. The structure of waterhouseite has no equivalent, although the unit-cell parameters reveal some relations with the two chemical analogues allactite, Mn₇(AsO₄)₂(OH)₈, and raadeite, M9₇(PO₄)₂ (OH)₈.