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1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ
2 Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD
3 Department of Earth Science, University of Manchester, Oxford Road, Manchester, M13 9PL, UK
* E-mail: jree99{at}esc.cam.ac.uk
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, ß = 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 +
1 = –2, 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 XM2 Mn and decrease in XM4 Mn 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: cummingtonite, order-disorder, cation partitioning, neuytron diffraction, phase transition
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