SHORT-RANGE ORDER IN AMPHIBOLES

Short-range order (SRO)is a prominent feature of many rock-forming minerals [e.g., amphiboles, micas, staurolite, scapolite]; it strongly affects the configurational entropy of solid solutions, and is important in the formation of activity models. The chemical composition of a mineral is the sum of all its short-range configurations; hence constraints on patterns of SRO can strongly affect chemical substitutions and solid-solution/immiscibility in minerals. When extended to deal with local atom arrangements in minerals, bond-valence theory indicates that SRO should be a prominent feature of solid solutions that involve heterovalent substitutions. Unfortunately, SRO is difficult to characterize in complex solid-solutions as diffraction is not sensitive to SRO, and those spectroscopic techniques that are sensitive [e.g., infrared spectroscopy in the principal OH-stretching region, MAS NMR] are not applicable to all minerals, and show compositional restrictions to their sensitivity. Nevertheless, IR has proved a powerful technique for characterizing SRO in OH-bearing minerals via the frequencies and intensities of fine-structure bands in the principal OH-stretching region. This has proved effective in amphibole solid-solutions, when the patterns of LRO (long-range order) are well-characterized [e.g., richterite-pargasite, tremolite-pargasite], and the results are in accord with local arrangements predicted by bond-valence arguments. Calculation of all possible SRO patterns in solid solutions allows assessment of the stability of each of the local arrangements by bond-valence theory; those arrangements that are not stable may limit the extent of solid solution. For example, along the tremolite-tschermakite join, tschermakitic compositions are constrained to be dominated by local arrangements that are not stable from a bond-valence perspective, accounting for the fact that natural and synthetic compositions along this join do not extend significantly past hornblende. In many complex oxysalt minerals, there are solid solutions involving heterovalent cations and heterovalent anions, e.g., Ti + 2O <=> Mg + 2OH. Generally, local bond-valence requirements are satisfied by local association of the substituents, coupled with significant positional disorder of both cations and anions.