Short-range order in minerals: amphiboles.
Résumé
Short-range order involves local clusters of atoms that occur either more or less frequently than predicted by a random distribution. Infrared spectroscopy in the principal (OH)-stretching region is sensitive to such local arrangements, and hence the (OH) group can be used as a local probe of short-range arrangements of atoms. Examination of natural amphiboles of fairly simple composition indicates that the principal (OH)-stretching frequency is sensitive to such local arrangements, generating fine structure that gives information on short-range arrangements. Moreover, the fine structure is sensitive to both nearest-neighbor and next-nearest-neighbor arrangements. The short-range arrangements that can occur are constrained by the local version of the valence-matching principle, and this helps in assignment of the bands in the fine structure of the corresponding infrared spectra. Recent results on synthetic amphiboles illustrate these issues. Monoclinic amphiboles in the systems richterite–pargasite, tremolite–pargasite and tremolite–hornblende show strong SRO involving T-, C- and A-group cations. Amphiboles involving (OH)–F solid-solution with ∅ (vacancy) at the A site (e.g., tremolite–fluorotremolite) show one-mode behavior, whereas amphiboles with Na or K at the A site (e.g., richterite–fluororichterite) show two-mode behavior, indicating that nearest-neighbor arrangements of atoms couple through an occupied A site, but do not couple through a vacant A site. Furthermore, the relative band intensities in (OH)–F amphibole solid-solutions showing two-mode behavior indicate that (OH) and F are completely short-range disordered with respect to each other in the amphibole series examined thus far. Amphiboles in the system pargasite–fluoropargasite show strong SRO of (OH) and F with regard to the cations occupying the associated nearest-neighbor M(1)M(1)M(3) sites: arrangements involving MgMgAl–(OH) are far more common than arrangements involving MgMgAl–F. Examination of Ti-bearing richteritic amphiboles show that [4]Ti4+ and Si are short-range disordered with regard to each other. Crystal-structure refinement, SIMS analysis and local bond-valence requirements suggest that [6]Ti4+ and O(3)O2– are locally associated at adjacent M(1) and O(3) sites in (at least some) amphiboles. It is apparent that SRO is very common in monoclinic amphiboles. Although much work remains to be done to fully characterize SRO in amphiboles, the general features are already emerging, and local bond-valence requirements seem to be the (principal) factor controlling this type of order. SRO is of significance in that it will affect the stability of amphiboles (and other minerals in which it occurs) through its entropy (and enthalpy) effects; the way in which these effects can be formulated for such a complicated case is not yet clear, but what is clear is that future thermodynamic models need to consider SRO in amphiboles and probably in other minerals in which heterovalent substitutions are common.