Iron distribution and structural order in synthetic calcic amphiboles studied by Moessbauer spectroscopy and Hrtem

Temperature‐dependen ctation distribution among the four nonequivalent M(1) to M(4) positions in synthetic calcic amphibole has been studied by means of Mössbauer spectroscopy. Both stoichiometric and Ca‐deficient compositions, with low Fe/Mg ratios, have been synthesized and annealed using hydrothermal techniques. Structural homogeneity of the run products was confirmed by use of high‐resolution transmission electron microscopy (hrtem).Results from Mössbauer spectroscopy reveal that Fe²⁺ is always present on the M(4) site, including the results for the stoichiometric composition that was expected to show the M(4) site entirely occupied by Ca. With decreasing temperature, Fe²⁺ increases on the M(4) site, decreases on the M(2) site, and remains constant on the M(1) + M(3) sites. The exchange of Fe²⁺ between the M(2) and M(4) sites is probably coupled with an exchange of Mg in the opposite direction by the disordering reaction $FeM(4)2++MgM(2)=MgM(4)+FeM(2)2+$⁠, in analogy with earlier studies of natural tremolite‐actinolite. The HRTEM studies reveal a high degree of structural order in the synthetic calcic amphiboles, in contrast to earlier work on synthetic Fe‐Mg‐Mn amphiboles where high concentrations of chain-width disorder and stacking faults have been found. These results imply that the composition influences the amount of structural imperfections found in synthetic amphiboles.