The crystal structure of a Mn-Mg amphibole from Nsuta, Ghana, with the composition (Ca0.24Mn2.41Mg4.20Fe0.15)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+.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.