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

Intracrystalline distribution of ferrous and ferric iron within pumpellyites ( (super VII) W (sub 8) (super VI) X (sub 4) (super VI) Y (sub 8) (super IV) Z (sub 12) O (sub 56-n) (OH) (sub 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 (super 57) Fe Moessbauer 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 Fe (sub 2) O (sub 3) of 10.01+ or -1.69 and 16.07+ or -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 (super X) [Mg (sub 0.298) Fe (sub 0.298(5)) Al (sub 0.405) ] (super Y) [Fe (sub 0.191) Al (sub 0.809) ] and (super X) [Mg (sub 0.244) Fe (sub 0.42(1)) Al (sub 0.34) ] (super Y) [Fe (sub 0.32) Al (sub 0.68) ], respectively. The Fe (super 2+) :Fe (super 3+) ratio of the CLW pumpellyite, determined using Moessbauer spectroscopy, is 12(1):88(3). By combining the average chemical composition data, the site occupancies at the X and Y sites, and the Fe (super 2+) :Fe (super 3+) ratio of the CLW pumpellyite, the chemical formulae of CLW and CHG pumpellyites are constructed as (Ca (sub 7.96) K (sub 0.02) Na (sub 0.01) ) (sub Sigma 7.99) (Mg (sub 1.19) Mn (super 2+) (sub 0.09) Fe (super 2+) (sub 0.39) Fe (super 3+) (sub 0.71) Al (sub 1.62 ) ) (sub Sigma 4.00) (Al (sub 6.47) Fe (super 3+) (sub 1.50) V (sub 0.02) Ti (sub 0.01) ) (sub Sigma 8.00) Si (sub 12.26) O (sub 43.33) (OH) (sub 12) .67 and (Ca (sub 8.01) K (sub 0.01) ) (sub Sigma 8.02) (Mg (sub 0.97) Mn (super 2+) (sub 0.02) Fe (super 2+) (sub 0.63) Fe (super 3+) (sub 1.03) Al (sub 1.19) ) (sub Sigma 4. 01) (Al (sub 5.44) Fe (super 3+) (sub 2.55) V (sub 0.01) ) (sub Sigma 8.00) Si (sub 12.02) O (sub 42.69) OH) (sub 13.31) , respectively, implying that the proper name of both pumpellyites is pumpellyite-(Al). The intracrystalline distribution coefficients of Fe (super 3+) versus Al between the X and Y sites, K (sub D) = (Fe (super 3+) /Al) (super X) /(Fe (super 3+) /Al) (super Y) , are 1.62 and 1.90 for the CLW and CHG pumpellyites, respectively, implying stronger X-site preference of Fe (super 3+) than Al (super 3+) . 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 Aa, respectively, which are larger than <X-O> = 2.040 and <Y-O> = 1.944 Aa 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) Aa, beta = 97.473(2) degrees with V = 1004.9(2) Aa (super 3) and are larger than those of the CLW pumpellyite, a = 8.8456(4), b = 5.9393(2), c = 19.1613(8) Aa, beta = 97.461(3) degrees with V = 998.14(7) Aa (super 3) .