New data on ilmajokite.

Ilmajokite, ideally Na10-x(Ce,Ba)xTi5Si14O22(OH)44, is a rare mineral, which occurs as granular deposits, crusts, and brushes of crystals up to 2 mm long on the walls of cavities in the central natrolitic zone of pegmatites in the Lovozero Tundra, Kola Peninsula, near the valley of the Ilmajok River. It has a bright yellow color but turns turbid and white when drying. It is optically biaxial positive and has oblique cleavage. It was described by [1,2] being the solely data available up to now. These authors described the mineral as probably monoclinic with a ~ 23, b ~ 24.4, c ~ 37 Å, while Anthony et al. [3] report a = 39.80(4), b = not determined, c = 29.83(5) Å, β = 96°38', C2/c or Cc. Up to now the structure of ilmajokite is unknown. Although it occurs as apparently crystalline material, it almost does not show any diffraction when powdered [1]. In order to obtain some diffraction information, Bussen et al. [1] glued several single crystals but the diffraction at high angles did remain very weak. Ilmajokite has a large quantity of H2O (24.54 wt % [1]), which releases on heating losing up to 12.7% at 175 °C, although a 6.5% is still retained at 320 °C. Water release starts as low as 60 °C but it is not complete until 760° C [1]. H speciation was determined by IR finding broad absorptions at 1618 and 3389-2889 cm-1 [1]. We collected diffraction data on a single crystal (0.10 x 0.08 x 0.04 mm) of ilmajokite provided by E. Semenov in 2004. We performed a data collection at room temperature by means of CCD-Oxford single-crystal diffractometers using both molybdenum and copper radiation. Diffraction experiments yielded very weak data and no diffraction over 3 Å-1. Cell refinement yielded a C-centered monoclinic cell with a = 35.774(4), b = 27.407(3), c = 31.131(5) Å, β = 95.66(1)°, V = 30374(7) Å3. Rint was very high (~32%) and no model could be obtained from these data. We performed therefore a low-temperature data collection at 125 K using a Bruker-AXS diffractometer with a SMART CCD detector and an OXFORD Cryostream 600 low temperature device. Diffraction was slightly enhanced up to ca. 2.8 Å-1. Cell refinement confirmed a C-centered monoclinic cell with a = 35.32(16), b = 26.93(12), c = 30.68(14) Å, β = 95.84(2)°, V = 29034(403) Å3. Internal agreement factor integrating up to 2.8 Å-1 was better (~17%) but still very high. We observed a volume reduction of up to a 4.5%, being more significant in the shorter lattice value (up to 1.8%), while the monoclinic angle shows a very faint increment. Structure solution by Direct Methods and Charge Flipping methods [4] have not been successful. Thus, the structure solution of this mineral awaits further experiments. References. [1] Bussen, I. V. , Gannibal, L. F., Goiko, E. A., Mer'kov, A. N., Nedorezova, A. P. (1972): Zapiski Vses Mineral Obshch, 101, 75-79; [2] Goiko, E.A., Bussen, I.V., Gannibal, L.F., Lipatova, E.A. (1974): Uch. Zap. Leningr. Gos. Univ., Ser.Biol. Nauk, 278, 174-181; [3] Anthony, J.W., Bideaux, R.A., Bladh, K.W., Nichols, M.C. (1995) Handbook of Mineralogy. Vol. II. Silica, Silicates. Mineral Data Publishing, Tucson, Arizona; [4] Palatinus L., Chapuis G. (2007): J. Appl. Cryst., 40, 786-790.