The Chemical form of Mn(II) and V(IV) in Mineral Phases as Determined by EPR Spectroscopy

Abstract

Electron paramagnetic resonance (EPR) spectroscopy is an analytical tool for the identification of paramagnetic cations associated with minerals. The assignment of an EPR signal obtained from a natural system, such as soils, to a specific mineral phase or a specific chemical form is often ambiguous because an EPR signal reflects the ligand field of a paramagnetic center that can occur in different mineral phases and chemical forms, and secondly EPR signals of different cations can be superimposed. An experimental approach that combines EPR spectroscopy with thermoanalytical methods was developed in order to assign paramagnetic cations unambiguously to a specific chemical form or to specific mineral phases in multimineral systems (Gehring and Karthein 1990; Gehring et al, 1993a). This approach is based on the assumption that different chemical forms or different mineral phases in a multimineral system differ in thermal stability. The purpose of this research was to apply this methodological approach to determine the chemical form of Mn(II) and V(IV) in a vermiculite sample (VTX-1) from Texas obtained from the Source Clay Depository of the Clay Minerals Society. Apart from vermiculite, magnesite was found in the sample by X-ray diffraction (XRD). Details of the EPR experiments are described elsewhere (Gehring and Karthein, 1990). The EPR spectrum of the untreated sample exhibited two groups of resonances around g=4.3 and g=2, respectively (Fig.l). The low-field resonance at g = 4.3 is characteristic for Fe(III) structure-bound in phyllosilicates (cf. Meads and Malden, 1975) and therefore can be assigned to the vermiculite. The spectrum around g = 2 showed a superposition of an eight-line and a six-line hyperfine-split (HFS) signal (Fig.2), that can be attributed to V(IV) and Mn(II), respectively. The Mn(II) signal is characterized by a gvalue of 2.002-t-0.001, and a hyperfine coupling constant A--9.44__+0.03mT. The HFS feature of the V(IV) spectrum contained both parallel and perpendicular components. For the parallel c o m p o n e n t , g l t = 1 . 9 4 1 _ _ + 0 . 0 0 1 , AII=18.11+0.07mT, and for the perpendicular c o m p o n e n t g_l_ = 1 . 9 8 5 _ _ + 0 . 0 0 1 , A / = 7.05_ 0.02mT were obtained. These parameters are in excellent agreement with McBride (1990). The Mn(II) and V(IV) could be associated with vermiculite and/or magnesite, either structure-bound, as adsorbate or as cluster. The latter possibility can be excluded because clusters would lead to broad signals without HFS.