Distribution of Fe among octahedral sites and its effect on the crystal structure of pumpellyite |
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| Authors: | Mariko Nagashima Takahiro Ishida Masahide Akasaka |
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| Institution: | (1) Department of Materials Creation and Circulation Technology, Graduate School of Science and Engineering, Shimane University, 690-8504 Matsue, Japan |
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| Abstract: | Two pumpellyites with the general formula W
8
X
4
Y
8
Z
12O56-n
(OH)
n
were studied using 57Fe Mössbauer spectroscopic and X-ray Rietveld methods to investigate the relationship between the crystal chemical behavior of iron and structural change. The samples are ferrian pumpellyite-(Al) collected from Mitsu and Kouragahana, Shimane Peninsula, Japan. Rietveld refinements gave Fe(X):Fe(Y) ratios (%) of 41.5(4):58.5(4) for the Mitsu pumpellyite and 46(1):54(1) for the Kouragahana pumpellyite, where Fe(X) and Fe(Y) represent Fe content at the X and Y sites, respectively. The Mössbauer spectra consisted of two Fe2+ and two Fe3+ doublets for the Mitsu pumpellyite, and one Fe2+ and two Fe3+ doublets for the Kouragahana pumpellyite. In terms of the area ratios of the Mössbauer doublets and the Fe(X):Fe(Y) ratios determined by the Rietveld refinements, Fe2+(X):Fe3+(X):Fe3+(Y) ratios are determined to be 22:14:64 for the Mitsu pumpellyite and 27:8:65 for the Kouragahana pumpellyite. By applying the Fe2+:Fe3+-ratio determined by the Mössbauer analysis and the site occupancies of Fe at the X and Y sites given by the Rietveld method together with chemical analysis, the resulting formula of the Mitsu and Kouragahana pumpellyites are established as Ca8(Fe
0.88
2+
Mg0.68Fe
0.77
3+
Al1.66)Σ3.99(Al5.67Fe
2.34
3+
)Σ8.01Si12O42.41(OH)13.59 and Ca8(Mg1.24Fe
0.65
2+
Fe
0.46
3+
Al1.66)Σ4.01(Al6.71Fe
1.29
3+
)Σ8.00Si12O42.14(OH)13.86, respectively. Mean Y–O distances and volumes of the YO6 octahedra increase with increasing mean ionic radii, i.e., the Fe3+→Al substitution at the Y site. However, change of the sizes of XO6 octahedra against the mean ionic radii at the X site is not distinct, and tends to depend on the volume change of the YO6 octahedra. Thus, the geometrical change of the YO6 octahedra with Fe3+→Al substitution at the Y site is essential for the structural changes of pumpellyite. The expansion of the YO6 octahedra by the ionic substitution of Fe3+ for Al causes gradual change of the octahedra to more symmetrical and regular forms. |
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| Keywords: | Pumpellyite Iron X-ray Rietveld analysis Mö ssbauer spectroscopy |
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