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 Cr
2O
3 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 Fe
2O
3 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 Fe
3+ 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
0.43Cr
3+0.09Fe
3+0.48]
M3Si
3O
12(OH) for CrEp1, [Ca]
A1[Ca
0.90Sr
0.10]
A2[Al
0.86Cr
3+0.14]
M1[Al]
M2[Al
0.34Cr
3+0.17Fe
3+0.49]
M3Si
3O
12(OH) for CrEp2, and [Ca]
A1[Ca
0.88Sr
0.12]
A2[Al
0.90Cr
3+0.10]
M1[Al]
M2[Al
0.37Cr
3+0.12Fe
3+0.51]
M3Si
3O
12(OH) for CrEp3. The selective distribution of Cr
3+ at the
M3 site in CrEp1, and the
KD values (
KD = (Cr
3+/Al)
M1/(Cr
3+/Al)
M3) of 0.35(3) for CrEp2 and 0.41(3) for CrEp3 show that the preference of Cr
3+ in the octahedral sites is M3 > M1 » M2. The variations in the
M1-O
i and
M3-O
i distances indicate an equidimensional expansion of the
M1O
6 and
M3O
6 octahedra by the substitution of Cr
3+(+ Fe
3+) for Al
3+.
View full abstract