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

Abstract

The ideal formula for chevkinite can be expressed as A 4BC 2D 2Si 4O 22. 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 Fe 3+/ΣFe ratio was 39.2%. A significant increase of Fe 3+ 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 Fe 2+ 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 Fe 3+ in the C and D sites, respectively. The simplified formula can be revised as: Ce 4Fe 2+ (Ti, Fe 2+, Fe 3+) 2(Ti, Fe 3+) 2Si 4(O, OH) 22. It indicated that the non-metamict chevkinite-(Ce) belonged to Fe 2+ end member of the chevkinite group because Fe 2+ was the predominant component in the B site.