Interpretation du spectre infrarouge de varietes isotopiques de pyropes synthetiques.

Three samples of pyrope, Mg3Al2Si3O12, 26Mg3Al2Si3O12, and Mg3Al230Si3O12 synthesized at 1000oC and 25 kbar were studied by IR spectroscopy. The use of isotopic species shows that the bands in the range 1100-500 cm-1 are shifted with the isotopic substitution of Si but not with that of Mg, i.e. these bands are due to vibrations of SiO4 tetrahedra. The bands below 500 cm-1, however, are shifted with the isotopic substitution of Si or Mg; these bands are therefore due not to specific vibrations for SiO4 or Mg2+ but must be assigned to complex vibrations involving simultaneous participation of SiO4, Mg2+, and probably Al3+. -R.A.H.