The oxidation state and distribution of Fe in pumpellyite from the Northern Chichibu Belt in the Hijikawa district, western Shikoku, Japan

Abstract

AbstractIntracrystalline distribution of ferrous and ferric iron within pumpellyites (VIIW8VIX4VIY8IVZ12O56–n(OH)n, Z = 1) from low-grade metamorphic green rocks of the Kanogawa unit in the Northern Chichibu Belt, Hijikawa district, western Shikoku, Japan, was investigated using electron microprobe analysis, X-ray Rietveld refinement, and 57Fe Mössbauer spectroscopic analysis to verify the regularity of the distribution of Fe within the octahedral X and Y sites in pumpellyite and its effect on the pumpellyite structure. Two Fe-rich pumpellyite samples, labelled CLW and CHG, with average total Fe2O3 of 10.01±1.69 and 16.07±1.08 wt.%, respectively, were separated from the rock samples. The site occupancies at the X and Y sites in the CLW and CHG pumpellyites, refined using powder X-ray diffraction data, are X[Mg0.298Fe0.298(5)Al0.405]Y[Fe0.191Al0.809] and X[Mg0.244Fe0.42(1)Al0.34]Y[Fe0.32Al0.68], respectively. The Fe2+:Fe3+ ratio of the CLW pumpellyite, determined using Mössbauer spectroscopy, is 12(1):88(3). By combining the average chemical composition data, the site occupancies at the X and Y sites, and the Fe2+:Fe3+ ratio of the CLW pumpellyite, the chemical formulae of CLW and CHG pumpellyites are constructed as (Ca7.96K0.02Na0.01)Σ7.99(Mg1.19Mn2+0.09Fe2+0.39Fe3+0.71Al1.62)Σ4.00(Al6.47Fe3+1.50V0.02Ti0.01)Σ8.00Si12.26O43.33(OH)12.67 and (Ca8.01K0.01)Σ8.02(Mg0.97Mn2+0.02Fe2+0.63Fe3+1.03Al1.19)Σ4.01(Al5.44Fe3+2.55V0.01)Σ8.00Si12.02O42.69OH)13.31, respectively, implying that the proper name of both pumpellyites is pumpellyite-(Al). The intracrystalline distribution coefficients of Fe3+ versus Al between the X and Y sites, KD = (Fe3+/Al)X/(Fe3+/Al)Y, are 1.62 and 1.90 for the CLW and CHG pumpellyites, respectively, implying stronger X-site preference of Fe3+ than Al3+. In the CHG pumpellyite, where the Fe contents at the X and Y sites are higher than in the CLW pumpellyite, the mean <X–O> and <Y–O> distances are 2.06 and 1.98 Å, respectively, which are larger than <X–O> = 2.040 and <Y–O> = 1.944 Å in the CLW pumpellyite. The unit-cell parameters of the CHG pumpellyite are a = 8.8672(3), b = 5.9562(2), c = 19.1899(6) Å, β = 97.473(2)° with V = 1004.9(2) Å3 and are larger than those of the CLW pumpellyite, a = 8.8456(4), b = 5.9393(2), c = 19.1613(8) Å, β = 97.461(3)° with V = 998.14(7) Å3.