Use of L-edge X-ray absorption spectroscopy to characterize multiple valence states of 3d transition metals; a new probe for mineralogical and geochemical research

Abstract

2p (L _2,3) X-ray absorption spectra are presented for a range of minerals to demonstrate the usefulness of L-edge spectroscopy as a symmetry- and valenceselective probe. 2p XAS provides a sensitive fingerprint of the electronic states of 3 d transition metals and can be applied to phases containing mixtures of such elements. Calculated spectra for 3d ⁿ → 2p ⁵ 3d ⁿ⁺¹ transitions provide a basis for the interpretation of the measured spectra. Thus, in principle, multiple valence states of a particular 3 d metal can be precisely characterized from a single L-edge spectrum. Examples of vanadium L-edge spectra are presented for a range of minerals; these complex spectra hold information concerning the presence of vanadium in multiple valence states. The Cu L-edge spectrum of sulvanite (Cu₃ VS₄) indicates the presence of both Cu⁺ and Cu²⁺; the V L-edge spectrum of the same sample shows that both V²⁺ and V⁵⁺ are present. Spectral simulations representing mixtures of Fe d ⁵ and Fe d ⁶ states are used to quantify Fe³⁺/∑Fe in a spinel, a glass, and an amphibole, all of which contain Fe as a major component. To illustrate the sensitivity of 2p XAS in a dilute system, the Fe L-edge spectrum of amethyst (α-SiO₂: Fe) has been recorded; this spectrum shows that ∼68% of the Fe in amethyst is Fe²⁺, and ∼32% is Fe³⁺. Although previous studies on amethyst using other spectroscopic methods cite evidence for Fe⁴⁺, there is no indication in the L-edge spectrum for Fe⁴⁺ in amethyst. Comparison of theoretical and experimental spectra not only allows the valence states of 3 d ions to be recognised, but also provides site-symmetry information and crystal field parameters for each ion site.