Effects of tetrahedral isomorphic substitution on the IR spectra of synthetic fluorine micas

Effects of tetrahedral isomorphic substitution on the IR spectra of fluorine micas were demonstrated by comparing the spectrum of taeniolite KMg 2 Li(Si 4 O 10 )F 2 with those of the substituted analogues, in which Ga, Al, B or Ge substituted for Si in tetrahedral sites. The e 1 1 bands move towards lower frequencies as the content of substituted cation in tetrahedral site increases, whereas The a 1 1 and a 1 2 bands moves towards higher frequencies. Linear relationships were found between a 1 2 the band frequency and the extent of tetrahedral substitutions. The magnitude of shifts per molar substitution for the a 1 2 bands is dependent on the species of substituted cation and decreases in the order Ga > Al > Ge > B, showing an intimate correlation with the field strength z 1 ·z 2 /r 2 M–O of the substituted cations. This implies that the polarizing power of the substituted cations has a pronounced effect on the Si-O stretching vibrations.