THE OPTICAL ABSORPTION SPECTRA OF SOME ANDRADITES AND THE IDENTIFICATION OF THE 6A1→4A14E(G) TRANSITION IN OCTAHEDRALLY BONDED Fe3+

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Abstract
The optical absorption spectra of andradites from three different localities are reported. The spectra show a very sharp d–d band at 22 700 cm−1 (440 mμ) that has been assigned to electronic transitions to the first field-independent state (4A14E(G)) in octahedrally bonded Fe3+. A strong broad d–d absorption at 19 000 cm−1 in the high-Ti andradite (melanite) has been assigned to octahedrally bonded Ti3+. The andradites also show broad bands at 12 500 cm−1, 16 500 cm−1, and 21 400 cm−1 that are part of the Fe3+ spectrum.The andradite spectra have been compared to the spectrum of an almandine-pyrope. The conclusion is reached that the Fe3+ octahedra in andradite and the pyralspite garnets are different. There is no justification, therefore, in giving an almandine-pyrope (that contains Fe3+) an andradite content on the basis of Fe3+ analyses. The Fe3+ atoms in pyralspites are better regarded as impurity ions.