Short-range order in amphiboles: A bond-valence approach

Short-range order describes the tendency for some local arrangements of atoms to be more frequent than a random distribution of atoms would indicate. Bond-valence theory indicates that local arrangements that closely obey the valence-sum rule should be more stable, and hence favored, over arrangements that lead to significant departures from the valence-sum rule. This argument should be particularly effective in predicting local arrangements in amphiboles because of the wide dispersion of bond valences associated with the different heterovalent site-occupancies characteristic of ideal end-members of the amphibole group. Of the amphibole end-member charge-arrangements, only tremolite, glaucophane, (alumino-)leakeite and ungarettiite have ordered distributions of cations; all other end-members involve disorder on two or more cation sites, affording extensive possibilities for short-range order. Here, the most probable patterns of short-range order have been derived for all monoclinic calcic, sodic-calcic and alkali amphiboles by examining local bond-valence distributions and their agreement with the valence-sum rule. For most structures, there is one set of patterns that agrees quite well with the valence-matching principle, and all other sets of patterns show significant deviations from the valence-matching principle. For some structures, one pattern of the best set of patterns of order deviates significantly from the valence-sum rule (e.g., tschermakite), and compositions close to these end-members are rare or non-existent. These patterns of local order will hopefully aid in the interpretation of experimental data, which are affected significantly by shea-range order in the amphibole structure.