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

The crystal structure of a manganoan cummingtonite, composition ([M4])(Na0. 13Ca0.41Mg0.46Mn1.00) 102,614(2)(n) from Talcville, New York, has been refi ned at high temperature using in situ neutron powder diffraction. The P2(1) /m to C2/m phase transition, observed as spontaneous strains +epsilon(1) = -epsilon(2), occurs at similar to 107 degrees C. Long-range: disordering be tween Mg2+ and Mn2+! on the M(4) and M(2) sites occurs above 550 degrees C. Mn2+ occupies the M(4) and M(2) sites preferring M(4) with a site-preferen ce energy of temperatures. Upon cooling, the ordered states of cation occup ancy are 'frozen in' and strains in lattice parameters are maintained, sugg esting that re-equilibration during cooling has not taken place.