Abstract
Abstract
Intracrystalline 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.
Subject
Geochemistry and Petrology
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