Allanpringite, Fe3(PO4)2(OH)35H2O, a new ferric iron phosphate from Germany, and its close relation to wavellite

Abstract

Allanpringite is a new ferric iron phosphate with the ideal formula Fe3(PO4)2(OH)3·5H2O, and is closely related to wavellite, Al3(PO4)2(OH)3·5H2O. Type locality is the dump of the abandoned Grube Mark near Essershausen, ca. 5 km SE of Weilburg/Lahn, Taunus, Hesse, Germany. The mineral occurs as pale brown-yellow, 010] acicular, invariably twinned (by non-merohedry) crystals which are always intergrown to form bundles of subparallel individuals. The maximum length of crystals is ca. 1.5 mm (usually much smaller); bundles can reach up to about 2 mm. The mineral is associated with beraunite (reddish “oxiberaunite” variety), cacoxenite, strengite and cryptomelane. Allanpringite is translucent to transparent, its streak is white with a pale yellowish tint, and it has a vitreous lustre. It shows a perfect cleavage parallel to the morphological elongation and one good cleavage parallel to {010}. It is brittle and has an uneven fracture, a Mohs hardness of ~3, and D (meas.) = 2.54(2) g/cm3, D (calc.) = 2.583 g/cm3 (for empirical formula). Optically, it is biaxial positive, with α = 1.662(5), β = 1.675(5), γ = 1.747(5), 2 Vγ (calc.) = 48°; pleochroism is strong: X colourless, Y colourless, Z dark yellow; absorption Z>>X~Y; orientation XYZ = ** b (pseudo-orthorhombic); no visible dispersion. Electron microprobe analysis yielded (wt.%): Fe2O3 47.84, Al2O3 0.34, Mn2O3 0.04; CuO 0.08, P2O5 28.56, F 0.02, H2Ocalc 23.49, less O≡F 0.01, total 100.36. The empirical formula is (Fe2.98Al0.03)(PO4)2(OH3.02F0.01)·4.97H2O, based on 16 O atoms. Allanpringite is monoclinic, space group P 21/ n , with a = 9.777(3), b = 7.358(2), c = 17.830(5) A, β = 92.19(4)°, V = 1281.7(6) A3 (single-crystal data) and Z = 4. Strongest lines in the X-ray powder diffraction pattern are [ d (A), I , hkl ]: 8.90 (100) (002), 8.41 (60) (10-1, 101), 5.870 (50) (110), 3.600 (50) (021, 02-1), 3.231 (80) (204, 12-2). The crystal structure has been determined using single-crystal X-ray diffraction data (Mo K α radiation, CCD area detector) obtained from a twinned fragment ( R ( F ) = 13.3%). The structure of allanpringite is a monoclinically distorted, pseudo-orthorhombic variant of the orthorhombic structure of its Al-analogue wavellite, Al3(PO4)2(OH,F)3·5H2O. Chains of corner-sharing, distorted Fe(O,OH,H2O)6 octahedra parallel to [010] are corner-linked by PO4 tetrahedra. Channels, also parallel to [010], host a positionally split water molecule. Average Fe-O distances of the three non-equivalent Fe atoms range between 2.014 and 2.021 A. Single-crystal laser-Raman spectroscopy confirms an overall weak hydrogen bonding scheme. The structure of allanpringite is also related to those of kingite and mitryaevaite. The amorphous santabarbaraite has a chemical formula basically identical to that of allanpringite. The name honours Dr. Allan Pring, eminent Australian mineralogist.