X-ray Rietveld and ~(57)Fe Moessbauer study of babingtonite from Kouragahana, Shimane Peninsula, Japan

摘要

Babingtonite from Kouragahana, Shimane Peninsula, Japan, was investigated using electron microprobe, X-ray Rietveld, and ~(57)Fe Moessbauer spectral analyses to characterize its chemical compositions, crystal structure, oxidation state of Fe, and distribution of Fe between two crystallographically independent octahedral Fe1 and Fe2 sites. The Kouragahana babingtonite occurs as single parallelohedrons with {100}, {001}, {001}, {111}, {110}, and {TOl} and sometimes shows penetration twinning. Both normal and sector-zoned crystals occur. Babingtonite crystals with sector zoning consist of sectors relatively enriched in Fe and of sectors enriched in Mg, Mn, and A1. Babingtonite also shows compositional zoning with higher Fe~(2+) and A1 core and higher Fe~(3+) and Mn rim. The average Fe content of the babingtonite without sector zoning is similar to the Fe~(2+)-rich sector of the sector-zoned babingtonite. The chemical formula based on the average composition of all analytical data (n = 193) is [Na_(0.01(2))Ca_(2.01(2))] [Mg_(0.11(4))Mn_(0.09(3))Feo~(2+)_(0.76(7))Fe~(3+)_(0.93(5))Ti_(0.01(1))Al_(0.06(5))]Si_(5.04(4))O_(14)(OH). X-ray Rietveld refinement was carried out using a model of space group P1. The result of the refinement is characterized byR-weighted pattern = 9.91, R-expected pattern = 6.37, and goodness-of-fit = 1.56. The unit cell parameters are a = 7.4667(3),b= 11.6253(6), c =6.6820(2)A, α = 91.533(4), β=93.886(3), γ= 104.203(4)°, and V=560.43(4) A~3. The refined site occupancies of atoms in the Fel and Fe2 sites are [Fe0.91(2)Mg0.09] and [Fe0.91(2) A10.09], respectively, if Mg is assumed to distribute in the Fe1 site and A1 in the Fe2 site. By allocating Mn, 0.09 atoms per formula unit (a.p.f.u.), to the Fel site, the site populations in the Fe1 and Fe2 sites are determined as [Fe0.82(2)Mn0.09Mg0.09] and [Fe0.91(2)A10.09] a.p.f.u., respectively. The ~(57)Fe Mossbauer spectrum taken at room temperature consists of three peaks, which were resolved into two doublets assigned to Fe~(2+) and Fe~(3+) at the two octahedral sites. The Fe~(2+):Fe~(3+) ratio was determined as Fe~(2+):Fe~(3+)= 43.3(3):56.7(4) and 47.1(4): 52.9(3) by applying two fitting models, and the average Fe~(2+):Fe~(3+) ratio was 45.2(4):54.8(4). The results of X-ray Rietveld analysis and Mossbauer spectroscopy indicate that Fe~(2+) and Fe~(3+) are ordered at the Fe1 and Fe2 sites, respectively, in the Kouragahana babingtonite.