Abstract
The electric field gradient at the octahedral sites in biotite is calculated by use of the point charge model for the ideal crystal structure. The contribution from the overlap distortions of cation orbitals by the surrounding ligands is included. The calculated values of the ferric quadrupole splittings are in good agreement with those obtained from Mossbauer spectra of micas. The effect of the neighbouring point defects and of real structure features of biotite on the electric field gradient are computed. It has been found that various point defects in adjacent sites produce varying values of ferric quadrupole splittings, which exceed the difference between the ideal parameters at the two distinct octahedral sites. A new interpretation of the biotite Mossbauer spectra is suggested.
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