| Summary: | 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 ...
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