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Allanpringite, Fe₃(PO₄)₂(OH)₃·5H₂O, a new ferric iron phosphate from Germany, and its close relation to wavellite

¹ Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstr. 14, 1090 Wien, Austria
² Ruhr-Universität Bochum, Zentrale Elektronen-Mikrosonde, Universitätsstr. 150, 44801 Bochum, Germany
³ Merzbachstr. 6, 52249 Eschweiler, Germany

^* Corresponding author, e-mail: uwe.kolitsch{at}univie.ac.at

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 = 1.662(5), ß = 1.675(5), = 1.747(5), 2V(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, H₂O_calc 23.49, less OF 0.01, total 100.36. The empirical formula is (Fe_2.98Al_0.03)(PO₄)₂(OH_3.02F_0.01)·4.97H₂O, 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, ß = 92.19(4)°, V = 1281.7(6) ų (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 (MoK 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 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.

Key-words: allanpringite, new mineral, ferric iron phosphate, crystal structure, wavellite.