Raadeite, Mg-7(PO4)(2)(OH)(8): a new dense-packed phosphate from Modum (Norway)

The new mineral raadeite, Mg-7(PO4)(2)(OH)(8), was found in nodules of apat ite + Mg-phosphates within the Tingelstadtjern serpentinite body, Modum dis trict, Norway. It occurs as veinlets, a few tens of mum wide, crosscutting althausite crystals; also as rare, up to 150 mum large anhedral inclusions in holtedahlite; and with apatite, althausite and magnesite in fibrous coro nae replacing heneuite. It is colourless, transparent, biaxial (-); for lam bda = 589 nm, n(x) 1.5945(5), n(y) 1.6069(5), n(z) 1.6088(5), 2V(meas) 45.6 (1)degrees, 2V(calc) 43 degrees; strong dispersion r > v, n(y) // b. The in frared spectrum shows prominent OH stretching bands at 3375, 3475, 3540 and 3580 cm(-1). Electron-microprobe analyses indicate near-end-member composi tion, with minor As, Fe and Mn. Raadeite could be synthesized from 3 to 15 kbar, 500 to 700 degreesC. It is monoclinic, space group P2(1)/n, a = 5.250 (1), b = 11.647(2), c = 9.655(2) Angstrom, beta = 95.94(1)degrees, Z = 2. S ingle-crystal structure study (R = 0.021) shows raadeite to be isostructura l with the Mn-arsenate allactite and could localize the four protons. The s tructure 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 simil ar hydrous Mg-phosphates on the join brucite-Mg-3(PO4)(2) (olivine-type str ucture:) can be formally compared with the polysomatic humite series and ot her 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-phospha tes.