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Fundamental Mossbauer parameters of synthetic Ca-Mg-Fe pyroxenes

M. Darby Dyar, Rachel L. Klima, Alexandra Fleagle and Samantha E. Peel
Fundamental Mossbauer parameters of synthetic Ca-Mg-Fe pyroxenes
American Mineralogist (July 2013) 98 (7): 1172-1186


Understanding of Fe site occupancy across the Ca-Fe-Mg pyroxene quadrilateral requires knowledge of space groups and appreciation of the diversity of site geometries across Ca-Mg-Fe composition space. Most commonly, site occupancies are measured using some combination of single-crystal structure refinements (SREF) from X-ray diffraction data and Mossbauer spectroscopy for bulk measurements. The vast majority of previous Mossbauer studies have been hampered by the lack of differential recoil-free fraction data that describe how the Fe (super 2+) and Fe (super 3+) cations are bonded in the M1 and M2 sites in pyroxene. To remedy this situation, this paper examines 658 Mossbauer spectra acquired from 64 synthetic samples covering the pyroxene quadrilateral in roughly 10 mol% increments, and determines their fundamental Mossbauer parameters as a function of composition. Results show variations in all the Mossbauer parameters studied: center shift (delta ), quadrupole splitting (Delta ), area, recoil-free fraction (f), Mossbauer temperature (Theta (sub M) ), and intrinsic isomer shift (delta (sub I) ). The most systematic variations with composition are seen for delta and f, while small variations are seen for Delta , Theta (sub M) , and delta (sub I) . These data are then related to characteristics of the pyroxene crystal structure to examine the relationship between site geometry and recoil-free fraction. In general, smaller bond lengths (e.g., in the M1 site along the enstatite-ferrosilite join) result in higher f values. As Ca is added to the structure and Mg is removed, the f value for M1 increases as the site becomes larger and more regular. Larger sites with lower bond strengths result in lower values of f because the cation is less tightly bound in the crystal structure and thus encounters more recoil. This result is in keeping with theoretical expectations, but has not previously been clearly demonstrated for minerals with experimental data. Values of recoil-free fraction determined in this study will facilitate more accurate determinations of cation site occupancies in pyroxenes from Mossbauer data and lend insights into the geometries of the M1 and M2 sites.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 98
Serial Issue: 7
Title: Fundamental Mossbauer parameters of synthetic Ca-Mg-Fe pyroxenes
Affiliation: Mount Holyoke College, Department of Astronomy, South Hadley, MA, United States
Pages: 1172-1186
Published: 201307
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 45
Accession Number: 2013-061389
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: Johns Hopkins University, USA, United States
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: 201337
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