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Mössbauer spectroscopy of phyllosilicates: effects of fitting models on recoil-free fractions and redox ratios

¹ Department of Astronomy, Mount Holyoke College, 50 College Street, South Hadley, MA 01075, USA, ² Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA, and ³ SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043 USA

^* E-mail: mdyar{at}mtholyoke.edu

(Received 16 April 2007; revised 20 November 2007)

Clay minerals are ubiquitous constituents in soils on Earth, are occasionally found in meteorites, and may also occur on planetary surfaces in the presence of water. However, little is known about the fundamental Mössbauer parameters (the intrinsic isomer shift, _I, the characteristic Mössbauer temperature, _M, and the recoil-free fraction, f) that are characteristic of clay minerals and critical to the correct interpretation of the Fe³⁺/Fe ratios as well as the mineral modes. Spectra of well characterized single mineral samples at multiple temperatures may be used for the determinations of f. Hence, measurements of five-layer silicates with a range of layer types are presented here: nontronite, Fe-smectite, glauconite, annite and biotite. The spectra were fitted using three different software packages: WMOSS from Science, Engineering & Education Co. in Minnesota; Recoil, from the University of Ottawa in Canada; and two programs used at the University of Ghent in Belgium. Four different approaches to modelling line shapes were used: (1) Lorentzian; (2) pseudo-Voigt (convolution of Lorentzian and Gaussian curves); (3) quadrupole-splitting distributions (QSD); and (4) a technique that does not assume a particular line shape (subsequently referred to as ‘model-independent’). Values of _I, _M and f were determined using the method of De Grave & Van Alboom (1991).

Results show that multiple doublets are routinely required by all models to represent Fe-site occupancy, even when all the Fe atoms of the same valence are in the same site, as is the case for dioctahedral smectite, nontronite, mica and glauconite. Consistent values of centre shift () and quadrupole splitting () were obtained for the two distributions of ^M2Fe³⁺ in the smectites. In glauconite, a single Fe²⁺ doublet was clearly resolved and gave systematic values for , and area, but the two Fe³⁺ doublets were less defined. In annite, two Fe²⁺ and two Fe³⁺ doublets were modelled, while three Fe²⁺ and one Fe³⁺ doublet were used for biotite. Three different programs that use Lorentzian line shapes gave very similar results for , and area. The two different implementations of QSD line shapes gave similar but sometimes slightly different results, and the pseudo-Voigt and model-independent fits usually fell between the ranges for Lorentzian and QSD results.

The value of _I is ~0.58 mm/s for Fe³⁺ and ~1.31 mm/s for Fe²⁺ across all models and line shapes, which is expected because the Fe³⁺ has an additional shielding 3d electron. Values for _M data are nearly identical for Fe³⁺ in nontronite and Fe-smectite (~450 K), somewhat varied for Fe³⁺ in glauconite and biotite (_M = ~730 K and ~615 K, respectively), and relatively distinct for Fe²⁺ (~350 K). Some values for _M and f could not be determined due to the non-monotonic behaviour of the fitted values for as a function of temperature. Values of f₂₉₅ were 0.821–0.917 for Fe³⁺ and 0.662–0.743 for Fe²⁺, consistent with previous studies of the recoil-free fraction in micas and other silicates. Calculated scatter in , , area and f values as a function of different line shapes and computer software was significantly reduced at lower temperatures. Sources of error in each of the calculated parameters are discussed.

KEYWORDS: Mössbauer, biotite, annite, smectite, nontronite, glauconite, recoil-free fraction, quadrupole-splitting distributions

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