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

Abstract

Abstract The crystal structure of a manganoan cummingtonite, composition [M4] (Na 0.13 Ca 0.41 Mg 0.46 Mn 1.00 ) [M1,2,3] (Mg 4.87 Mn 0.13 )(Si 8 O 22 )(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 P 2 1 / m to C 2/ m phase transition, observed as spontaneous strains +ε 1 = −ε 2 , occurs at ˜107°C. Long-range disordering between Mg 2+ and Mn 2+ on the M(4) and M(2) sites occurs above 550°C. Mn 2+ 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 X Mn M2 and decrease in X Mn M4 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.