Optical spectroscopy study of natural Fe, Ti-bearing calcic amphiboles

Over thirty samples of natural Ti-bearing amphiboles with Ti- and Fe-conten ts ranging from 0.111 to 0.729 atom per formula unit (a.p.f.u.) and from 0. 479 to 2.045 a.p.f.u.. respectively, were studied by means of optical absor ption spectroscopy and microprobe analysis. Thirteen samples were also stud ied by Mossbauer spectroscopy. A strong pleochroic absorption edge, causing the dark brown colours of Ti-bearing amphiboles, is attributed to ligand-m etal and metal-metal charge transfer transitions involving both iron and ti tanium ions (O2- - -> Fe3+, Fe2+, O2- --> Ti4+ and Fe2+ + Ti4+ --> Fe3+ + T i3+). A broad intense Y-polarized similar to 22000 cm(-1) (nu(1/2) approxim ate to 3700 cm(-1)) in spectre of two low iron amphiboles with a relatively low Fe3+/ Fe-total ratio, both from eclogite-like rocks in kimberlite xeno liths, was attributed to electronic Fe2+(M3)+ Ti4+(M2) --> Fe3+(M3) + Ti-3(M2) IVCT transitions. The IVCT bands of other possible ion pairs, involvin g Ti4+ and Fe2- in M2 and M1, M4 sites, respectively, are presumed to be at higher energies, being obscured by the absorption edge.