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

Abstract

The new mineral raadeite, Mg 7(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 µ m wide, crosscutting althausite crystals; also as rare, up to 150 µ m large anhedral inclusions in holtedahlite; and with apatite, althausite and magnesite in fibrous coronae replacing heneuite. It is colourless, transparent, biaxial (- ); for l = 589 nm, n x 1.5945(5), n y 1.6069(5), n z 1.6088(5), 2V meas 45.6(1)° , 2V calc 43° ; strong dispersion r > v, n y // b. The infrared spec- trum shows prominent OH stretching bands at 3375, 3475, 3540 and 3580 cm -1. Electron-microprobe analyses indi- cate 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) A , b = 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 PO 4 tetra- hedra. Raadeite and similar hydrous Mg-phosphates on the join brucite- Mg 3(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.