The identification of Fe^(2+) in the M(4) site of calcic amphiboles

Evidence is presented for the occurrence of Fe^(2+) in the predominantly Ca-filled M(4) site of calcic amphiboles. Absorption bands in the electronic spectra at 1030 nm in β and at 2470 in a provide a sensitive means for identifying Fe^(2+) in this site. The large є value for the 1030 band, the large energy separation between both bands, and the similarity to the spectra of Fe^(2+) in large, highly distorted coordination sites in other minerals provide the basis for the M(4) site assignment. The energy splittings and polarization anisotropy of these bands are analyzed using a point-charge model based on C_(2v) symmetry. Spectral data indicate that the Fe^(2+) content in the M(4) site varies significantly among the calcic amphiboles