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

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

Source: European Journal of Mineralogy, Volume 13, Number 2, 1 April 2001 , pp. 319-327(9)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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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

m wide, crosscutting althausite crystals; also as rare, up to 150

m large anhedral inclusions in holtedahlite; and with apatite, althausite and magnesite in fibrous coronae replacing heneuite. It is colourless, transparent, biaxial (-); for lambda

= 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 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, beta

= 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.

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Keywords: RAADEITE; NEW MINERAL; MAGNESIUM PHOSPHATE; CRYSTAL STRUCTURE; ALLACTITE; COLLINSITE; MODUM DISTRICT; NORWAY

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/01/0013-0319

Publication date: 2001-04-01

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