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

Abstract

Over thirty samples of natural Ti-bearing amphiboles with Ti- and Fe-contents 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 absorption spectroscopy and microprobe analysis. Thirteen samples were also studied by Mössbauer spectroscopy. A strong pleochroic absorption edge, causing the dark brown colours of Ti-bearing amphiboles, is attributed to ligand-metal and metal-metal charge transfer transitions involving both iron and titanium ions (O²⁻→ Fe³⁺, Fe²⁺, O²⁻→ Ti⁴⁺ and Fe²⁺ + Ti⁴⁺→ Fe³⁺ + Ti³⁺). A broad intense Y-polarized band ∼22 000 cm⁻¹ (ν_1/2 ≈ 3700 cm⁻¹) in spectra of two low iron amphiboles with a relatively low Fe³⁺/Fe_total ratio, both from eclogite-like rocks in kimberlite xenoliths, was attributed to electronic Fe²⁺(M3) + Ti⁴⁺(M2)→Fe³⁺(M3)+Ti³⁺(M2) IVCT transitions. The IVCT bands of other possible ion pairs, involving Ti⁴⁺ and Fe²⁺ in M2 and M1, M4 sites, respectively, are presumed to be at higher energies, being obscured by the absorption edge.