ALLANPRINGITE, FE3(PO4)2 (OH)3·5H2O, 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 Weillburg/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), 2Vγ(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. %): Fe2 O347.84,Al2O30.34, Mn2O30.04;CuO 0.08, P2O5 28.56, F0.02, H2Ocalc 23.49, less O≡F 0.01, total 100.36. The empirical formula is (Fe2.98 Al0.03)(PO4)2(OH3.02 F0.01)·4.97 H2O, based on 16 O atoms. Allanpringite is monoclinic, space group P21/n, with a = 9.777(3), b = 7.358(2), c = 17.830(5) Å, β=92.19(4)°, V=1281.7(6) Å3 (single-crystal data) and Z = 4. Strongest lines in the X-ray powder diffraction pattern are [d (Å), 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 (MoKα 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)3·5 H2O. 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 Å. 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. © 2006 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart.