Fe-bearing prehnite, Ca
2Al
2-pFe
pSi
3O
10(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 Fe
3+ 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
(Rwp) = 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
Rwp = 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 Fe
3+ 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 Fe
3+ 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.
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