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A ⁵⁷Fe MÖSSBAUER SPECTROSCOPIC STUDY OF SUGILITE, KNa₂(Fe³⁺,Mn³⁺,Al)₂Li₃Si₁₂O₃₀

Charles A. Geiger

Institut für Geowissenschaften, Abteilung Mineralogie, Universität Kiel, Olshausenstr. 40, D–24098 Kiel, German

E-mail address: chg{at}min.uni-kiel.de

Sugilite, ideally KNa₂(Fe³⁺,Mn³⁺,Al)₂Li₃Si₁₂O₃₀, from the Wessels mine, Kalahari, South Africa, was studied using ⁵⁷Fe Mössbauer spectroscopy. The spectra confirm published electron-microprobe data and X-ray single-crystal results, which indicate that Fe³⁺ is ordered at the A octahedral site. The magnitude of the quadrupole splitting, E_Q for Fe³⁺ at both 84 K and room temperature is 0.00 mm/s. This value is very low for a silicate; reported E_Q values for octahedrally coordinated Fe³⁺ are generally greater than 0.20 mm/s. Octahedron distortion, calculated from published X-ray-diffraction data, gives an octahedron-angle variance of 7 and a mean elongation of the octahedron of 1.002. This indicates nearly regular octahedral geometry, and hence a symmetric electric-field gradient around Fe³⁺. A crystal-chemical interpretation for the regular octahedral geometry and the resulting low E_Q value for Fe³⁺ in the Mössbauer spectrum of sugilite is that structural strain is largely "taken up" by weak Li–O bonds. They permit highly distorted LiO₄ tetrahedra, whose variance in the O–Li–O angle, 502, and mean elongation of the tetrahedron, 1.148, are among the largest observed values for inorganic crystal structures. Weak Li–O bonds allow the edge-shared, more strongly bonded Fe³⁺O₆ octahedra to remain regular in geometry.

Keywords: sugilite, ring silicate, Mössbauer spectroscopy, iron, hyperfine parameters, site distortion.