Raadeite, Mg (sub 7) (PO (sub 4) ) (sub 2) (OH) (sub 8) ; a new dense-packed phosphate from Modum (Norway)

The new mineral raadeite, Mg (sub 7) (PO (sub 4) ) (sub 2) (OH) (sub 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 mu m wide, crosscutting althausite crystals; also as rare, up to 150 mu 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 (sub x) 1.5945(5), n (sub y) 1.6069(5), n (sub z) 1.6088(5), 2V (sub meas) 45.6(1) degrees , 2V (sub calc) 43 degrees ; strong dispersion r>v, n (sub y) //b. The infrared spectrum shows prominent OH stretching bands at 3375, 3475, 3540 and 3580 cm (super -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 degrees C. It is monoclinic, space group P2 (sub 1) /n, a = 5.250(1), b = 11.647(2), c = 9.655(2) Aa, beta = 95.94(1) degrees , 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 (sub 4) tetrahedra. Raadeite and similar hydrous Mg-phosphates on the join brucite-Mg (sub 3) (PO (sub 4) ) (sub 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.