⁵⁷Fe Mössbauer and X-ray Rietveld studies of ferrian prehnite from Kouragahana, Shimane Peninsula, Japan

抄録

Fe-bearing prehnite, Ca₂Al_2-pFe_pSi₃O₁₀(OH)₂ (Z=2), from Kouragahana, Shimane Peninsula, Japan, was investigated using ⁵⁷Fe Mössbauer spectroscopy and X-ray Rietveld method. Iron and aluminum contents of the prehnite are variable, with p ranging from 0.003 to 0.425 (n=97). Fe-poor and Al-rich prehnites generally grow over clusters of Fe-rich prehnite crystals. Intergrowth texture of Fe-poor and Fe-rich crystals is also common in spherical aggregates. The ⁵⁷Fe Mössbauer spectrum consists of one doublet with isomer shift = 0.360 mm/s, quadrupole splitting = 0.276 mm/s and peak width = 0.310 mm/s. The doublet is assigned to Fe³⁺ in the octahedral site. X-ray Rietveld refinement was carried out using two structural models of space groups Pmna and Pma2, Results of the refinement are characterized by R-weighted pattern (R_wp) = 9.30 %, ‘Goodness-of-fit’ indicator (S) = 1.276 and Durbin-Watson statistic d (D-W d) of 1.485 for the refinement in space group Pmna, and by R_wp = 10.00 %, S = 1.367 and D-W d of 1.383 in space group Pma2, indicating that the fit of the former refinement is better than the latter. The refined Fe occupancies at the octahedral site in space groups Pmna and Pma2 are 0.16 ± 0.01 and 0.20 ± 0.01, respectively. In the Pmna-structure, the T2 site is occupied by Si_0.5Al_0.5 with disordering scheme, and average T2-O distance is 1.668 Å. In contrast, in the Pma2-structure, Al and Si are distributed in an ordered state in the T2 site, and average Al_T2-O and Si_T2-O distances are 1.72 and 1.61 Å, respectively. Absence of Fe³⁺ from the tetrahedral site may be attributed to the small size for the cation and inflexible character of the T2 site. A very small quadrupole splitting of the Mössbauer doublet by Fe³⁺ in the octahedral site suggests the octahedral site has a highly symmetrical site-geometry. This is consistent with a more symmetrical environment in the centrosymmetric space group Pmna.