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Accurate determination of ferric iron in garnets by bulk Moessbauer spectroscopy and synchrotron micro-XANES

M. Darby Dyar, Elly A. Breves, Erica Emerson, Samuel W. Bell, Melissa Nelms, Marie V. Ozanne, Samantha E. Peel, Marco L. Carmosino, Jonathan M. Tucker, Mickey E. Gunter, Jeremy S. Delaney, Antonio Lanzirotti and Alan B. Woodland
Accurate determination of ferric iron in garnets by bulk Moessbauer spectroscopy and synchrotron micro-XANES
American Mineralogist (October 2012) 97 (10): 1726-1740

Abstract

Measurements of Fe (super 3+) /Sigma Fe in geological materials have been intractable because of lack of access to appropriate facilities, the time-consuming nature of most analyses, and the lack of precision and reproducibility in most techniques. Accurate use of bulk Mossbauer spectroscopy is limited by largely unconstrained recoilless fraction (f), which is used to convert spectral peak area ratios into valid estimates of species concentrations and is unique to different mineral groups and compositions. Use of petrographic-scale synchrotron micro-XANES has been handicapped by the lack of a consistent model to relate spectral features to Fe (super 3+) /Sigma Fe. This paper addresses these two deficiencies, focusing specifically on a set of garnet group minerals. Variable-temperature Mossbauer spectra of the Fe (super 2+) -bearing almandine and Fe (super 3+) -bearing andradite end-members are used to characterize f in garnets, allowing Fe (super 3+) /Sigma Fe to be measured accurately. Mossbauer spectra of 19 garnets with varying composition were acquired and fit, producing a set of garnet-specific standards for Fe (super 3+) analyses. High-resolution XANES data were then acquired from these and 15 additional previously studied samples to create a calibration suite representing a broad range of Fe (super 3+) and garnet composition. Several previously proposed techniques for using simple linear regression methods to predict Fe (super 3+) /Sigma Fe were evaluated, along with the multivariate analysis technique of partial least-squares regression (PLS). Results show that PLS analysis of the entire XANES spectral region yields the most accurate predictions of Fe (super 3+) in garnets with both robustness and generalizability. Together, these two techniques present reliable choices for bulk and microanalysis of garnet group minerals.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 97
Serial Issue: 10
Title: Accurate determination of ferric iron in garnets by bulk Moessbauer spectroscopy and synchrotron micro-XANES
Affiliation: Mount Holyoke College, Department of Astronomy, South Hadley, MA, United States
Pages: 1726-1740
Published: 201210
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 70
Accession Number: 2013-001138
Categories: Mineralogy of silicatesGeneral geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Secondary Affiliation: University of Massachusetts, USA, United StatesUniversity of Idaho, USA, United StatesRutgers University, USA, United StatesUniversity of Chicago, USA, United StatesUniversitaet Frankfurt, DEU, Germany
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201301
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