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

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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.

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

Document Type: Research Article

Publication date: 01 April 2001

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  • The European Journal of Mineralogy publishes original papers, review articles and letters dealing with the mineralogical sciences s.l. These include primarily mineralogy, petrology, geochemistry, crystallography and ore deposits, as well as environmental, applied and technical mineralogy. Nevertheless, papers in any related field, including cultural heritage, will be considered.
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