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Mn-Mg disordering in cummingtonite: a high-temperature neutron powder diffraction study

Published online by Cambridge University Press:  05 July 2018

J. J. Reece ,
S. A. T. Redfern ,
M. D. Welch  and
C. M. B. Henderson
J. J. Reece*
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ
S. A. T. Redfern
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ
M. D. Welch
Affiliation:
Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD
C. M. B. Henderson
Affiliation:
Department of Earth Science, University of Manchester, Oxford Road, Manchester, M13 9PL, UK
*
*E-mail: [email protected]
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Abstract

The crystal structure of a manganoan cummingtonite, composition [M4](Na0.13Ca0.41Mg0.46Mn1.00) [M1,2,3](Mg4.87Mn0.13)(Si8O22)(OH)2, (Z = 2), a = 9.5539(2) Å, b = 18.0293(3) Å, c = 5.2999(1) Å, β = 102.614(2)° from Talcville, New York, has been refined at high temperature using in situ neutron powder diffraction. The P21/m to C2/m phase transition, observed as spontaneous strains +ε1 = −ε2, occurs at ˜107°C. Long-range disordering between Mg2+ and Mn2+ on the M(4) and M(2) sites occurs above 550°C. Mn2+ occupies the M(4) and M(2) sites preferring M(4) with a site-preference energy of 24.6±1.5 kJ mol−1. Disordering induces an increase in XMnM2 and decrease in XMnM4 at elevated temperatures. Upon cooling, the ordered states of cation occupancy are ‘frozen in’ and strains in lattice parameters are maintained, suggesting that re-equilibration during cooling has not taken place.

Keywords

cummingtoniteorder-disordercation partitioningneuytron diffractionphase transition
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 2 , April 2000 , pp. 255 - 266
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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