Mn-Mg distribution in a C2/m manganoan cummingtonite; crystal-chemical considerations

The crystal structure of a Mn-Mg amphibole from Nsuta, Ghana, with the composition (Ca_0.24Mn_2.41Mg_4.20Fe_0.15)Si₈O₂₂(OH)₂, C2/m symmetry, unit-cell dimensions a = 9.605(1), b = 18.124(1), c = 5.325(1) Å, and ² = 102.64(1)° has been refined to an R factor of 0.047 on the basis of 1212 nonzero reflections measured on an automated X-ray diffractometer. Site-occupancy refinement indicates that the distorted octahedral M(4) site is nearly filled with Mn²⁺, whereas small amounts of Mn²⁺ also occur at the smaller octahedral M(1), M(2), and M(3) sites, the sequence of site preference being M(4)≫ M(1) > M(2) > M(3). The site preference of Mn²⁺ for the M(4) site in a Mn-Mg amphibole is nearly twice as strong as that of Fe²⁺ in the Fe-Mg amphibole because of a combination of the larger ionic size of Mn²⁺ and the greater covalency at the M(4) site compared to the M(1), M(2), and M(3) sites; the latter effect is indicated by short M(4)-O(4) bonds and the superhyperfine splitting in Mn²⁺ epr spectra from trace amounts of Mn²⁺ in tremolite. The compositional limit for the P2₁/m to C2/m phase transition at ambient temperatures is presumably close to Mn₂Mg₅Si₈O₂₂(0H)₂, at which the M(4) site is nearly filled with Mn²⁺.