X-ray Rietveld and ⁵⁷Fe Mössbauer study of babingtonite from Kouragahana, Shimane Peninsula, Japan

Abstract

Babingtonite from Kouragahana, Shimane Peninsula, Japan, was investigated using electron microprobe, X-ray Rietveld, and ⁵⁷Fe Mössbauer 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 {101} 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 Al. Babingtonite also shows compositional zoning with higher Fe²⁺ and Al core and higher Fe³⁺ and Mn rim. The average Fe content of the babingtonite without sector zoning is similar to the Fe²⁺-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)Fe²⁺_0.76(7)Fe³⁺_0.93(5)Ti_0.01(1)Al_0.06(5)]Si_5.01(4)O₁₄(OH). X-ray Rietveld refinement was carried out using a model of space group P1. The result of the refinement is characterized by R-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) Å, α = 91.533(4), β = 93.886(3), γ = 104.203(4)°, and V = 560.43(4) ų.
The refined site occupancies of atoms in the Fe1 and Fe2 sites are [Fe0.91(2)Mg0.09] and [Fe0.91(2)Al0.09], respectively, if Mg is assumed to distribute in the Fe1 site and Al in the Fe2 site. By allocating Mn, 0.09 atoms per formula unit (a.p.f.u.), to the Fe1 site, the site populations in the Fe1 and Fe2 sites are determined as [Fe0.82(2)Mn0.09Mg0.09] and [Fe0.91(2)Al0.09] a.p.f.u., respectively. The ⁵⁷Fe Mössbauer spectrum taken at room temperature consists of three peaks, which were resolved into two doublets assigned to Fe²⁺ and Fe³⁺ at the two octahedral sites. The Fe²⁺:Fe³⁺ ratio was determined as Fe²⁺:Fe³⁺ = 43.3(3):56.7(4) and 47.1(4):52.9(3) by applying two fitting models, and the average Fe²⁺:Fe³⁺ ratio was 45.2(4):54.8(4). The results of X-ray Rietveld analysis and Mössbauer spectroscopy indicate that Fe²⁺ and Fe³⁺ are ordered at the Fe1 and Fe2 sites, respectively, in the Kouragahana babingtonite.