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High-temperature order-disorder in (Fe0.5Mn0.5) (sub 2) SiO (sub 4) and (Mg0.5Mn0.5) (sub 2) SiO (sub 4) olivines; an in situ neutron diffraction study

Simon A. T. Redfern, C. Michael B. Henderson, Kevin S. Knight and Bernard J. Wood
High-temperature order-disorder in (Fe0.5Mn0.5) (sub 2) SiO (sub 4) and (Mg0.5Mn0.5) (sub 2) SiO (sub 4) olivines; an in situ neutron diffraction study (in EMPG-VI; Experimental mineralogy, petrology and geochemistry, E. A. J. Burke (editor), C. Chopin (editor), W. V. Maresch (editor), L. Ungaretti (editor) and C. Willaime (editor))
European Journal of Mineralogy (April 1997) 9 (2): 287-300

Abstract

Time-of-flight neutron powder diffraction has been used to determine the T-time dependence of M-site occupancies and crystal structures of these two synthetic olivines at up to 1000 degrees C. In both, Mn shows a preference for the M2 site at low T, but becomes increasingly disordered between M1 and M2 with increasing T. The equilibrium non-convergent disordering behaviour is described by a Landau expression for the free-energy change due to ordering. The activation energy for Fe-Mn exchange between M1 and M2 iscalculated to be 193 + or - 3 kJ/mol and for Mn-Mg exchange 172 + or - 3 kJ/mol. The M-site occupancy at room-T is shown to be a function of the cooling rate of the crystal, indicating the possibility that (Mg,Fe) (sub 2) SiO (sub 4) olivine might be useful as a geospeedometer for realtively rapid cooling events, such as the cooling of small high-level basaltic intrusions. The relationship between M-site ordering in these Mn-bearing samples and structural parameters such as <<<$I>M-O>><$I> bond lengths, octahedral bond-angle variance and cell parameters is described.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 9
Serial Issue: 2
Title: High-temperature order-disorder in (Fe0.5Mn0.5) (sub 2) SiO (sub 4) and (Mg0.5Mn0.5) (sub 2) SiO (sub 4) olivines; an in situ neutron diffraction study
Title: EMPG-VI; Experimental mineralogy, petrology and geochemistry
Author(s): Redfern, Simon A. T.Henderson, C. Michael B.Knight, Kevin S.Wood, Bernard J.
Author(s): Burke, E. A. J.editor
Author(s): Chopin, C.editor
Author(s): Maresch, W. V.editor
Author(s): Ungaretti, L.editor
Author(s): Willaime, C.editor
Affiliation: University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom
Pages: 287-300
Published: 199704
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
Meeting name: Sixth international symposium on Experimental mineralogy, petrology and geochemistry
Meeting location: Bayreuth, DEU, Federal Republic of Germany
Meeting date: 19960410April 10-13, 1996
References: 33
Accession Number: 1997-036943
Categories: Mineralogy of silicates
Document Type: Serial Conference document
Bibliographic Level: Analytic
Secondary Affiliation: University of Manchester, GBR, United KingdomISIS, GBR, United KingdomUniversity of Bristol, GBR, United Kingdom
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United KingdomTwickenhamUKUnited Kingdom
Update Code: 199712
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