Abstract: The use of optical absorption spectroscopy for the study of the changes in local distortions during phase transformations is discussed, with particular reference to the P2₁3-P2₁2₁2₁ transition in K₂Co₂(SO₄)₃ langbeinite. In order to extend this technique to the study of cobalt-doped cadmium langbeinite, K₂Cd₂(SO₄)₃, the consequences of the doping with regard to site occupancies and low signal strength are discussed, and some details of data modelling are considered. The problem of modelling the spectrum of Finero sapphirine is taken as an example, and it is shown that even simple and evidently inadequate models can provide useful information.

The results of optical spectroscopic investigation of the P2₁3-P2₁2₁2₁ transition in K₂(Cd_0.98Co_0.02)₂(SO₄)₃ langbeinite are presented. The trigonal splitting of the spin allowed ⁴T_1g(F)-⁴T_1g(P) transition has been determined, and shows a linear increase with increasing temperature in the high temperature cubic phase, and a constant value in the low temperature phase. This is in good agreement with previous structural work on this material, and it is concluded that optical spectroscopy of doped materials can provide useful information about site distortions.