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

摘要

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_8O_(22)(OH)_2, (Z=2), a=9.5539(2) A, B=18.0293(3) A, c= 5.2999(t) A, #beta#= 102.614(2)deg from Talcville, New York, has been refined at high temperature using in silo neutron powder diffraction. The P2_1/m to C2/m phase transition, observed as spontaneous strains +#epsilon_1#=-#epsilon#_2, occurs at approx 107 deg C. Long-range disordering between Mg~(2+) and Mn~(2+) on the M(4) and M(2) sites occurs above 550 deg C. MW 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 At'M4n 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.