Crystal chemistry and Raman spectroscopy of momoiite from Japan

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Marco E. Ciriotti
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Crystal chemistry and Raman spectroscopy of momoiite from Japan

Messaggio da Marco E. Ciriotti » mar 10 set, 2019 10:35

Prossima pubblicazione.

▪ Endo, S., Nagashima, M., Enami, M. (2019): Crystal chemistry and Raman spectroscopy of momoiite from Japan. Journal of Mineralogical and Petrological Sciences, 114, (in press).

Momoiite, a Mn2+– and V3+–dominant member of the garnet group, has been reported from metamorphosed manganese deposits in Japan. Near end–member momoiite (Mn3V2Si3O12) has not yet been found in nature, and the chemical composition of natural momoiite is written as (Mn,Ca)3(V,Al)2Si3O12, which represents intermediate compositions in solid–solution series between spessartine and Mn–rich goldmanite. Single–crystal X–ray diffraction data were obtained for an optically isotropic momoiite crystal from the Fujii mine, Fukui Prefecture, Japan. Structural refinement in the Ia3d space group converged to R1 = 1.45%, and the lattice constant is a = 11.8745(7) Å (Z = 8). The refined structural formula is X(Mn1.763Ca1.237)Σ3.000Y(V1.461Al0.539)Σ2.000ZSi3O12, which is consistent with electron microprobe analysis. Raman spectra of the solid–solution series between spessartine and Mn–rich goldmanite show continuous change in peak positions. The Si–O stretching and O–Si–O bending vibrations show downward frequency shifts with decreasing Mn/(Mn + Ca) and increasing V/(V + Al), which are associated with increase in mean ionic radii in the X and Y sites. The Al–V substitution controls the frequency shifts of the O–Si–O bending vibrations much more effectively than the Mn–Ca substitution. In contrast, the frequency shift of the SiO4 rotation mode shows strong correlation with the Mn–Ca substitution and is apparently independent on the Al–V substitution. This behavior suggests that the rotation of the SiO4 tetrahedron is strongly influenced by sharing two edges with XO8–dodecahedra. Crystal–chemical and Raman spectroscopic data of natural momoiite are consistent with those expected as an intermediate composition in the solid–solution series.
Marco E. Ciriotti

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