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Thermal expansion behaviour of orthopyroxenes; the role of the Fe-Mn substitution

L. Scandolo, M. L. Mazzucchelli, Matteo Alvaro, Fabrizio Nestola, F. Pandolfo and M. C. Domeneghetti
Thermal expansion behaviour of orthopyroxenes; the role of the Fe-Mn substitution
Mineralogical Magazine (February 2015) 79 (1): 71-87


Two Pbca orthopyroxene samples, donpeacorite (DP N.1) and enstatite (B22 N.60) with chemical formulae Mn (sub 0.54) Ca (sub 0.03) Mg (sub 1.43) Si (sub 2) O (sub 6) (X (sub Mn) = 0.27) and Fe (sub 0.54) Ca (sub 0.03) Mg (sub 1.43) Si (sub 2) O (sub 6) (X (sub Fe) = 0.27), respectively, were investigated by single-crystal X-ray diffraction at high-temperature conditions.The nearly identical X (sub Fe) and X (sub Mn) make the two samples the perfect candidates to investigate the effect of the compositional change at the M2 site (i.e. Fe-Mn substitution) on the thermal expansion behaviour of orthopyroxenes.Therefore, the unit-cell parameter thermal expansion behaviour of both samples has been investigated in the temperature range between room T and 1073 K. No evidence for phase transitions was found over that range. The two samples have been previously disordered with an ex situ annealing at approximately 1273 K.The unit-cell parameters and volume thermal expansion data, collected on the disordered samples, have been fitted to a Fei Equation of State (EoS) and the following coefficients obtained: V (sub 0) = 853.35(4) Aa (super 3) , alpha (sub V,303K) = 2.31(24) X 10 (super -5) K (super -1) and V (sub 0) = 845.40(6) Aa (super 3) , alpha (sub V,303K) = 2.51(25) X 10 (super -5) K (super -1) for DP N.1 and B22 N.60, respectively.While there is no difference in the volume thermal expansion coefficient as a function of composition and the expansion along the B: direction is nearly identical for both samples, slight differences have been found along A: and C: lattice directions. The thermal expansion along the A: direction is counterbalanced by that along C: being responsible for the changes in lattice expansion scheme from alpha (sub b) > alpha (sub c) > alpha (sub a) at room T, to alpha (sub c) > alpha (sub b) > alpha (sub a) at high T. Therefore, as a result of the different behaviour along a and c, the unit-cell volume thermal expansion for both samples is identical within estimated standard deviations. The negligible effect of the Fe-Mn substitution on the bulk thermal expansion can be applied when dealing with geothermobarometry based on the elastic host-inclusion approach (e.g. Nestola et al., 2011; Howell et al., 2010; Angel et al., 2014a,b, 2015). In fact, though the compressibility effect is still not known, the nearly identical thermal expansion coefficients will not affect the entrapment pressure (P (sub e) ).

ISSN: 0026-461X
EISSN: 1471-8022
Serial Title: Mineralogical Magazine
Serial Volume: 79
Serial Issue: 1
Title: Thermal expansion behaviour of orthopyroxenes; the role of the Fe-Mn substitution
Affiliation: Universita di Pavia, Dipartimento di Scienze della Terra e dell'Ambiente, Pavia, Italy
Pages: 71-87
Published: 201502
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 50
Accession Number: 2015-061805
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
Secondary Affiliation: Universita di Padova, ITA, Italy
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201527
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