SHORT-RANGE ORDER IN AMPHIBOLES - A BOND-VALENCE APPROACH

Short-range order describes the tendency for some local arrangements o f atoms to be more frequent than a random distribution of atoms would indicate. Bond-valence theory indicates that local arrangements that c losely obey the valence-sum rule should be more stable, and hence favo red, over arrangements that lead to significant departures from the va lence-sum rule. This argument should be particularly effective in pred icting local arrangements in amphiboles because of the wide dispersion of bond valences associated with the different heterovalent site-occu pancies characteristic of ideal end-members of the amphibole group. Of the amphibole end-member charge-arrangements, only tremolite, glaucop hane, (alumino-)leakeite and ungarettiite have ordered distributions o f cations; all other end-members involve disorder on two or more catio n sites, affording extensive possibilities for short-range order. Here , the most probable patterns of short-range order have been derived fo r all monoclinic calcic, sodic-calcic and alkali amphiboles by examini ng local bond-valence distributions and their agreement with the valen ce-sum rule. For most structures, there is one set of patterns that ag rees quite well with the valence-matching principle, and all other set s of patterns show significant deviations from the valence-matching pr inciple. For some structures, one pattern of the best set of patterns of order deviates significantly from the valence-sum rule (e.g., tsche rmakite), and compositions close to these end-members are rare or non- existent. These patterns of local order will hopefully aid in the inte rpretation of experimental data, which are affected significantly by s hea-range order in the amphibole structure.