Distribution of chromium among the octahedral sites in chromian epidote from Iratsu, central Shikoku, Japan

抄録

The crystal structures of chromian epidotes from the Sambagawa metamorphic rocks, central Shikoku, Japan, were refined using single-crystal data to investigate the distribution of chromium among the three independent octahedral sites. On the basis of the electron probe microanalysis of the chromian epidotes on the polished surface of thin sections, three chromian epidote grains in a thin section were selected for X-ray intensity data collection, and their Cr₂O₃ values were as much as 1.79, 4.99, and 4.74 wt%, respectively, which led to values of 0.113, 0.315, and 0.299 Cr apfu, respectively. Nevertheless, their Fe₂O₃ contents were nearly constant (8.1-8.7 wt%, 0.49-0.52 Fe apfu) and their SrO contents were not significant (1.8-3.2 wt%, 0.08-0.15 Sr apfu). The selected chromian epidote grains were picked out using a binocular optical microscope from the thin sections, and were labeled as CrEp1, CrEp2, and CrEp3, respectively. The oxidation state of chromium was found to be trivalent by the optical spectrum. ⁵⁷Fe Mössbauer doublets with isomer shift of 0.28-0.53 mm/sec and quadrupole splitting of 1.77-2.24 mm/sec indicate an exclusive distribution of Fe³⁺ at the M3 site. The crystal structure of CrEp1 [a = 8.8815(6), b = 5.5956(5), c = 10.1532(5) Å, β = 115.159(2)], CrEp2 [a = 8.9165(4), b = 5.6226(2), c = 10.1728(5) Å, β = 115.2365(9)], and CrEp3 [a = 8.895(2), b = 5.610(1), c = 10.146(2) Å, β = 115.177(2)] were refined using 1145, 1853 and 1438 unique reflections, respectively, and the calculations converged at R factors of 5.8%, 6.6%, and 3.2%, respectively. The resulting structural formula were: [Ca]^A1[Ca_0.86Sr_0.14]^A2[Al]^M1[Al]^M2[Al_0.43Cr³⁺_0.09Fe³⁺_0.48]^M3Si₃O₁₂(OH) for CrEp1, [Ca]^A1[Ca_0.90Sr_0.10]^A2[Al_0.86Cr³⁺_0.14]^M1[Al]^M2[Al_0.34Cr³⁺_0.17Fe³⁺_0.49]^M3Si₃O₁₂(OH) for CrEp2, and [Ca]^A1[Ca_0.88Sr_0.12]^A2[Al_0.90Cr³⁺_0.10]^M1[Al]^M2[Al_0.37Cr³⁺_0.12Fe³⁺_0.51]^M3Si₃O₁₂(OH) for CrEp3. The selective distribution of Cr³⁺ at the M3 site in CrEp1, and the K_D values (K_D = (Cr³⁺/Al)^M1/(Cr³⁺/Al)^M3) of 0.35(3) for CrEp2 and 0.41(3) for CrEp3 show that the preference of Cr³⁺ in the octahedral sites is M3 > M1 » M2. The variations in the M1-Oi and M3-Oi distances indicate an equidimensional expansion of the M1O₆ and M3O₆ octahedra by the substitution of Cr³⁺(+ Fe³⁺) for Al³⁺.