New data on S-bearing katoite from Tadano, Fukushima Prefecture, Japan: an implication of the presence of both (SO₃)^2- and (SO₄)^2- in the garnet structure

Abstract

Katoite, ideally denoted as {Ca₃}[Al₂](□_xSi_3-x)(OH)_4xO_12-4x, 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}[(Al_1.614Fe³⁺_0.208Mg_0.063Ti_0.019)_Σ1.90S⁴⁺_0.096]_Σ2.00(□_1.867Si_1.120S⁶⁺_0.013)_Σ3.00
[(OH)_7.192O_4.612F_0.147Cl_0.048]_Σ12.00.