Crystal Chemistry of Iron in Non-Metamict Chevkinite-(Ce): Valence State and Site Occupation Proportions

The ideal formula for chevkinite can be expressed as A4BC2D2Si4O22. It is important to determine the valence state and site occupation proportions for Fe among the B, C, and D octahedral sites as it may help to identify different species in the chevkinite group. Non-metamict chevkinite-(Ce) from Mianxi alkali feldspar-granite, Sichuan Province, China, was investigated using Mössbauer spectroscopy. The Fe3+/ΣFe ratio was 39.2%. A significant increase of Fe3+ occured during metamictization and annealing for chevkinite-group minerals. In metamict samples Fe tended to lower coordination. According to the correlation between bond length and isomer shift (IS), the quadrupole doublets with IS = 1.10 and 0.94 mm·s−1 can be assigned to Fe2+ in the B and C octahedral sites, respectively. Based on the correlation between octahedral distortion and quadrupole splitting (QS), the quadrupole doublets with QS = 0.86 and 0.77 mm·s−1 can be assigned to Fe3+ in the C and D sites, respectively. The simplified formula can be revised as: Ce4Fe2+ (Ti, Fe2+, Fe3+)2(Ti, Fe3+)2Si4(O, OH)22. It indicated that the non-metamict chevkinite-(Ce) belonged to Fe2+ end member of the chevkinite group because Fe2+ was the predominant component in the B site.