Original paper

Raadeite, Mg7(PO4)2(OH)8: a new dense-packed phosphate from Modum (Norway)

Chopin, Christian; Ferraris, Giovanni; Prencipe, Mauro; Brunet, Fabrice; Medenbach, Olaf

Abstract

The new mineral raadeite, Mg7(PO4)2(OH)8, was found in nodules of apatite + Mg-phosphates within the Tingelstadtjern serpentinite body, Modum district, Norway. It occurs as veinlets, a few tens of μm wide, crosscutting althausite crystals; also as rare, up to 150 μm large anhedral inclusions in holtedahlite; and with apatite, althausite and magnesite in fibrous coronae replacing heneuite. It is colourless, transparent, biaxial (-); for λ = 589 nm, nx 1.5945(5), ny 1.6069(5), nz 1.6088(5), 2Vmeas 45.6(1)°, 2Vcalc 43°; strong dispersion r > v, ny // b. The infrared spectrum shows prominent OH stretching bands at 3375, 3475, 3540 and 3580 cm-1. Electron-microprobe analyses indicate near-end-member composition, with minor As, Fe and Mn. Raadeite could be synthesized from 3 to 15 kbar, 500 to 700°C. It is monoclinic, space group P21/n, a = 5.250(1), b = 11.647(2), c = 9.655(2) Å, β = 95.94(1)°, Z = 2. Single-crystal structure study (R = 0.021) shows raadeite to be isostructural with the Mn-arsenate allactite and could localize the four protons. The structure consists of brucite-derived layers with 2/7 octahedral vacancies, which are linked by sharing a face and corners with interlayer Mg polyhedra (5+1 coordination) and corners with the PO4 tetrahedra. Raadeite and similar hydrous Mg-phosphates on the join brucite-Mg3(PO4)2 (olivine-type structure) can be formally compared with the polysomatic humite series and other dense hydrous Mg silicates on the brucite-forsterite join. However, the structural variety of the relevant phosphates cannot be accounted for by a polysomatic series, nor are they all high-pressure phases.The new mineral honours Gunnar Raade, for his contribution to the mineralogy of Mg-phosphates.

Keywords

raadeitenew mineralmagnesium phosphatecrystal structureallactitecollinsitemodum districtnorway