THE OPTICAL ABSORPTION SPECTRA OF SOME ANDRADITES AND THE IDENTIFICATION OF THE 6 A 1 → 4 A 1 4 E(G) TRANSITION IN OCTAHEDRALLY BONDED Fe 3+

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 ( 4 A 1 4 E(G)) in octahedrally bonded Fe 3+ . A strong broad d–d absorption at 19 000 cm −1 in the high-Ti andradite (melanite) has been assigned to octahedrally bonded Ti 3+ . 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 Fe 3+ spectrum.The andradite spectra have been compared to the spectrum of an almandine-pyrope. The conclusion is reached that the Fe 3+ octahedra in andradite and the pyralspite garnets are different. There is no justification, therefore, in giving an almandine-pyrope (that contains Fe 3+ ) an andradite content on the basis of Fe 3+ analyses. The Fe 3+ atoms in pyralspites are better regarded as impurity ions.