Water Molecules in the Channel-Like Cavities of Osumilite

Journal of MineralogicalWater molecules and Petrological in the channel Sciences,-like cavities Volume of 108, osumilite page 101─ 104, 2013 101 LETTER Water molecules in the channel-like cavities of osumilite * * ** Nozomi SOGAWA , Keiji SHINODA and Norimasa SHIMOBAYASHI *Department of Geosciences, Faculty of Science, Osaka City University, Sugimoto Sumiyoshi, Osaka 558-8585, Japan **Department of Geology and Mineralogy, Graduate School of Scinece, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan The crystal structure of osumilite is characterized by channel-like cavities that are composed of double six- membered rings along the c-axis, which is similar to the channels of beryl and cordierite. Beryl and cordierite generally include water molecules in the channel, and these have been extensively investigated using mainly infrared (IR) spectroscopy. Two major types of water molecules have been determined, however, the water molecules in the channel-like cavities of osumilite have not been reported. Polarized IR absorption spectra of water molecules in osumilite are presented here. The polarized IR absorption spectra of oriented osumilite that was hydrothermally treated at 600 °C and under 100 MPa for 72 h revealed three pairs of split peaks with equivalent −1 −1 −1 intensity. They can be assigned to ν3-I and ν1-I (3651 cm and 3554 cm ), ν3-IIa and ν1-IIa (3635 cm and 3601 −1 −1 −1 cm ), and ν3-IIb and ν1-IIb (3603 cm and 3551 cm ) as well as water molecules in the channels of cordierite and beryl. Keywords: Osumilite, Water molecule, IR spectroscopy INTRODUCTION orients both the H-H vector and molecular plane parallel to the c-axis, whereas type II H2O orients its H-H vector Osumilite is a tecto-silicate mineral with a hexagonal and molecular plane perpendicular to and parallel to the crystal system, belongs to milarite group. General chemi- c-axis. Although Goldman and Rossman (1978) presented cal formula is C M2 (T2)3 (T1)12 O30, where C = K, Na, M a polarized IR spectrum of biaxial osumilite, there have = Mg, Fe, T2 = Al, Fe, Mg, T1 = Si, Al. The crystal struc- been few reports on water molecules in the channel-like ture of osumilite is characterized by double six-membered cavities of osumilite. Natural osumilite does not generally rings that consist of T1 tetrahedra (Brown and Gibbs, include water molecules in the channel-like cavities, due 1969). The double six-membered rings are linked by T2 to its volcanic origin. However, from a spectroscopic in- tetrahedra along the c-axis. The C-sites between the dou- terest of the similar crystal structure of osumilite with ble six-membered rings are occupied by K or Na. Assum- beryl and cordierite, polarized IR spectra were measured ing that the C-sites lack cations, the double six-membered to reveal the characteristics of water molecules in the rings form a long cavity along the c-axis. The long cavity channel-like cavities of osumilite. Single crystals of os- is similar to the channel structure of cordierite and beryl. umilite (~ 3 mm diameter) were obtained from Tsukigase, When all C-sites are occupied by cations in a stoichio- Kawai of Hida City, Gifu Prefecture in Japan. Osumilite metric osumilite, no channel forms. However, ratio of the exhibits strong dichroism, where irradiation of a thin sec- C-site cations of natural osumilite is often below 1. In tion of osumilite with polarized light parallel to the c-axis these cases, channel-like cavities can form partially in the results in blue coloration under the E^c-axis and no col- osumilite structure. oration under the E//c-axis, where E is the electric vector There have been many reports on water molecules in of the polarized light. the channels of beryl and cordierite, of which two major types of H2O have been recognized and denoted type I EXPERIMENTAL and type II after Wood and Nassau (1967). Type I H2O Quantitative chemical analyses of the osumilite from the doi:10.2465/jmps.121019b same locality using an EPMA (JEOL JXA-8105) gave the K. Shinoda, [email protected] Corresponding author following chemical formula; (K0.70Na0.19Ca0.01Ba0.01)∑0.91 102 N. Sogawa, K. Shinoda and N. Shimobayashi Table 1. Chemical analysis of osumilite Figure 1. Polarized IR absorption spectra of natural osumilite under the (a) E//c-axis and (b) E^c-axis. Locality is Tsukigase, Kawai of Hida City, Gifu Prefecture in Japan. tal recovered from the gold tubes did not look affected. Thin sections of hydrothermally treated osumilite parallel (Fe1.17Mg0.70Mn0.11)∑1.98(Al2.67Mg0.32Ti0.01)∑3.00(Si10.42Al1.58) to the c-axes were also prepared with X-ray precession ∑12.00O30. The acceleration voltage was 15 kV, beam cur- camera. Oriented thin sections were prepared by polishing rent was 10 nA, and beam diameter was 3 μm. Table 1 both surfaces of the treated osumilite to expose the cores. shows weight percentage of oxides and mole ratios on the Strong dichroism when observed with a polarizing micro- basis of O = 30. The powder X-ray diffraction analysis scope remained after the hydrothermal treatments. Polar- using RIGAKU Smartlab-SS gave the cell parameters of ized IR spectra were measured using a Fourier transform a = 10.167 Å, and c = 14.327 Å. infrared (FTIR) spectrometer (Janssen Micro FTIR, Jas- An oriented thin section of natural osumilite with // co). c-axis was first prepared using an -X ray precession camera. The thickness of the thin section was 85 μm. Figures RESULTS and DISCUSSION 1a and 1b show polarized IR absorption spectra of the E// c- and E^c-axes at room temperature (RT) for natural os- The chemical analyses indicate that total C-sites cations umilite, respectively. There are no peaks in the absorption ratio was 0.91 below 1. This suggests that the C-sites are −1 range of H2O stretching modes between 4000 cm and partially vacant, and that some channel-like cavities pos- 3000 cm−1. The natural osumilite does not include water sibly form in the osumilite. Figures 2a and 2b show typi- molecules; therefore, H2O was introduced into the chan- cal polarized IR spectra at RT of osumilite treated with nels of osumilite using the same experimental technique 2μL H2O. The thickness of the //c-axis thin section was reported by Fukuda et al. (2009). Single crystals of osu- 121 μm. Five peaks (three sharp and two broad) were milite were sealed in gold tubes with various aliquots of identified in each spectrum and were separated by Lorent- water (2 μL, 5 μL, and 10 μL) and were held at 600 °C zian fitting. The peak center, height and full width at half and under a hydrostatic pressure of 100 MPa for 72 h in a maximum (FWHM) are listed in Table 2. Six sharp peaks −1 −1 −1 high pressure hydrothermal vessel. After annealing, crys- at 3651 cm , 3601 cm , and 3551 cm for E//c-axis, and Water molecules in the channel-like cavities of osumilite 103 Table 2. Results of peak separation and assignments peaks at ~ 3440 cm−1 and 3300 cm−1 were assigned to freely moving water molecules in the channels from the wavenumber, peak broadness and weak IR polarization. The bending mode of the H2O could not be observed in the IR absorption spectra, due to the overlap of the vibra- tion range of osumilite and the bending mode of the H2O. Clear relationship between OH absorbance and water ali- quot was not observed in this study. In the case of osumilite, three pairs of ν3 and ν1 peaks were observed in the wavenumber range of 3000-4000 cm−1 of which the peak height and FWHM were equivalent. In contrast, two peaks were observed for hexagonal beryl under the E//c-axis and only one intense peak and Figure 2. Polarized IR absorption spectra of hydrothermally treated some weak peaks were observed under the E^c-axis osumilite under the (a) E//c-axis and (b) E^c-axis. Peak separation was conducted by Lorentzian fitting. (Charoy et al. 1996). Charoy et al. (1996) reported an in- −1 tense peak at 3660 cm under the E^c-axis, which was assigned to ν3 of type II H2O, and some weak peaks at −1 −1 −1 −1 at 3635 cm , 3603 cm , and 3554 cm for E^c-axis can 3610-3630-3647 cm were assigned to ν1 of type I H2O. be assigned to type I H2O (I) and two type II H2O (IIa, IIb) For orthorhombic cordierite, two peaks were observed oriented in the channel-like cavities of osumilite as well under the E//c-axis, and an intense peak with some weak as water molecules in the channels cordierite and beryl by peaks was observed under the E//b-axis. An intense peak −1 following reasons. The IR absorption wavenumber of the at 3633 cm under E^c (E//b) is assigned to ν3 of type II −1 asymmetric stretching mode (ν3) is generally higher than H2O, and a small peak at 3595 cm is assigned ν1 of type the symmetric stretching mode (ν1), and the type I H2O in I H2O (Fukuda and Shinoda, 2011). In the case of cordier- the channels of beryl and cordierite has a higher absorp- ite and beryl, the absorption intensities for ν1 of type I are tion wavenumber than that for type II H2O (Fukuda and much weaker than the other three modes. In the case of Shinoda 2008). The IR active orientation for ν3 of type I osumilite, the six equivalent peaks are split and can be and ν1 of type II is the E//c-axis.

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