New data on S-bearing katoite from Tadano, Fukushima Prefecture, Japan: an implication of the presence of both (SO<sub>3</sub>)<sup>2−</sup> and (SO<sub>4</sub>)<sup>2−</sup> in the garnet structure

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  • BANNO Yasuyuki
    Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST)
  • MOMMA Koichi
    Department of Geology and Paleontology, National Museum of Nature and Science
  • MIYAWAKI Ritsuro
    Department of Geology and Paleontology, National Museum of Nature and Science
  • BUNNO Michiaki
    The University Museum, The University of Tokyo

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<p>Katoite, ideally denoted as {Ca3}[Al2](□xSi3−x)(OH)4xO12−4x, where 1.5 < x ≤ 3, was found in skarn xenoliths from Tadano, Fukushima Prefecture, Japan, and investigated by electron microprobe analyses, X-ray single crystal structure refinement, and infrared (IR) spectroscopy. Katoite characteristically contains up to 0.18 atoms per formula unit (apfu) of S and has a range of compositions, wherein Si = 0.96-1.25 apfu, Al = 1.47-1.74 apfu, Fe = 0.13-0.28 apfu, and Mg = 0.01-0.10 apfu. A difference Fourier map revealed the residual electron density near the octahedral Y site (Wyckoff position = 16a), and we interpreted that S with three-fold coordination occurs at the position of the residual electron density, which is represented as the Y’ site (Wyckoff position = 32e). The final refinement introduced the Y’ site to yield R1 [F2 > 2σ(F2)] = 0.0353 with space group Ia3d and unit cell parameter a = 12.24095(8) Å. The IR spectrum in the range of 800-1200 cm−1 shows a band at 1124 cm−1 due to ν3(SO4) and doublet bands at 879 and 931 cm−1 with different absorbances interpreted as ν3(SiO4) overlapping with ν3(SO3). The combined results of IR spectroscopy and structure refinement imply that in the Tadano katoite, S4+ and S6+, forming (SO3)2− and (SO4)2− coordinations, are placed at the Y’ and tetrahedral Z sites, respectively. Assuming that S is preferentially allotted to the Y’ site as S4+ to compensate for the deficiency of the octahedral cations Al, Fe, Mg, Mn, and Ti at the Y site, chemical formulae, including possible S4+ and S6+ contents, may be calculated. The average chemical formula for 14 different spots is as follows: {Ca3}[(Al1.614Fe3+0.208Mg0.063Ti0.019)Σ1.90S4+0.096]Σ2.00(□1.867Si1.120S6+0.013)Σ3.00[(OH)7.192O4.612F0.147Cl0.048]Σ12.00.</p>

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