The optical absorption coefficient of five different natural garnet crystals has been measured at 300 K over the wavenumber range from 1000 to 30 000 cm−1. Peaks in the absorption spectrum associated with OH, Fe2+, Fe3+, Mn2+, and Cr3+ ions have been identified. Pure Mg3Al2Si3O12 and Ca3Al2Si3O12 appear to have an intrinsic absorption coefficient below 1 cm−1 in the wavenumber range 3000–30 000 cm−1. There are intrinsic multiphonon lattice peaks at wavenumbers of about 1790 and 2600 cm−1 in all five garnets.
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The refractive index was measured at the Na D line at 5893 Å.
The five garnet crystals varied mainly with respect to the divalent-ion composition R in the formula R3Al2Si3O12. The table gives the fraction of the R lattice positions occupied by Ca, Fe, Mg, or Mn.
Table II
Survey of reported optical absorption bands of Cr, Fe, and Mn impurities in natural garnets at 300 K.
Dodec., oct., tetr. mean the ion is in a dodecahedral, octahedral, or tetrahedral site, respectively.
The host lattices are designated by the following terms. In case the crystal is a mixture of two or more simple garnets, the mixture is indicated. Alm =Almandite =Fe3Al2Si3O12; An = Andradite = Ca3Fe2Si3O12; G = Grossularite = Ca3Al2Si3O12; P = Pyrope = Mg3Al2Si3O12; S =Spessartite = Mn3Al2Si3O12; U =Uvarovite =Ca3Cr2Si3O12.
Table III
Survey of reported optical absorption bands of Cr, Fe, and OH in synthetic garnets at 300 K.
The refractive index was measured at the Na D line at 5893 Å.
The five garnet crystals varied mainly with respect to the divalent-ion composition R in the formula R3Al2Si3O12. The table gives the fraction of the R lattice positions occupied by Ca, Fe, Mg, or Mn.
Table II
Survey of reported optical absorption bands of Cr, Fe, and Mn impurities in natural garnets at 300 K.
Dodec., oct., tetr. mean the ion is in a dodecahedral, octahedral, or tetrahedral site, respectively.
The host lattices are designated by the following terms. In case the crystal is a mixture of two or more simple garnets, the mixture is indicated. Alm =Almandite =Fe3Al2Si3O12; An = Andradite = Ca3Fe2Si3O12; G = Grossularite = Ca3Al2Si3O12; P = Pyrope = Mg3Al2Si3O12; S =Spessartite = Mn3Al2Si3O12; U =Uvarovite =Ca3Cr2Si3O12.
Table III
Survey of reported optical absorption bands of Cr, Fe, and OH in synthetic garnets at 300 K.