Characterisation of synthetic trioctahedral micas by Mössbauer spectroscopy

Abstract

Trioctahedral potassium micas |K}[M₃]〈T₄〉O₁₀(OH)₂ have been synthesized by hydrothermal techniques with various cationic substitutions in the octahedral and the tetrahedral sheet. Taking annite |K}[Fe ²⁺₃ ]〈AlSi₃〉O₁₀(OH)₂ as the reference mineral, [Fe²] was replaced by [Mg²] and [Ni²], 〈Al³⁺〉 by 〈Fe³⁺〉 and finally [Fe²⁺] + 〈Si⁴⁺〉 by [Al³⁺] + 〈Al³⁺〉. Mössbauer spectra were evaluated in terms of quadrupole splitting distributions (QSDs) using three generalized sites for 〈Fe³⁺〉, [Fe³⁺] and [Fe²]. Annites, nominally free of 〈Fe³⁺〉, show a lower limit of [Fe³⁺]/Fe _tot of 0.10, which stabilizes the structure. The ferrous iron, [Fe²], QSD consists of two main components. In some of the solid solution series, there is strong experimental evidence for a third ferrous component, particularly at higher [Al³⁺] contents. This third component is centered at low quadrupole splittings and may be assigned to a defect [Fe²] site, forming 1:2 structures with two neighbouring trivalent octahedral cations. For charge compensation one OH⁻ is replaced by O²⁻ for each [M³⁺] cation. The ferrous QSDs vary systematically with chemical composition. Compared to those of annite, the QSD parameters (mean quadrupole splitting 〈QS〉 and quadrupole splitting with maximum probability, QS _peak ) are shifted towards higher values with increasing [Mg²] and [Ni²] contents, and decrease slightly with increasing content of trivalent cations. These trends can be interpreted in terms of changes in the local environment around the Fe probe nucleus, i.e., in terms of decreasing or increasing distortions from the ideal octahedral configurations.