- © The Mineralogical Society Of America
Extensive spectroscopic, diffraction and theoretical work in the last ten years has shown that short-range order (SRO) of both cations and anions is a common feature of amphiboles (Hawthorne et al. 2005). From an experimental perspective, the problem is that SRO does not obey the translational symmetry of the structure in which it occurs, and hence SRO is difficult to detect and decipher directly by standard diffraction methods. There is information on SRO resident in diffuse scattering from a crystal, but this information is quite difficult to extract, particularly when the SRO is complicated, and this approach has only been used for simpler materials. SRO can be detected by several spectroscopic techniques [e.g., infrared spectroscopy, magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy], but the problem is that only part of the local arrangement is derived, and the complete picture of SRO is often not clear. Here, we examine SRO of cations and anions in monoclinic amphiboles, and show that detailed information on SRO can be derived by a combination of infrared spectroscopy and local bond-valence theory, augmented by results from Rietveld and single-crystal diffraction.
Let us first examine the issue of LRO for a simple (two-dimensional) crystal of general composition M2SiO4 where M = Mg, Fe2+, and consider the composition Mg0.8 Fe2+1.2 SiO4 for the case where there are two distinct sites, M(1) and M(2), in the unit cell (Fig. 1⇓). Maximum order involves complete occupancy of one site by Fe2+ and occupancy of the other site by a mixture of Mg and Fe2+. Either M(1) or M(2) can be fully occupied by Fe2+, leading to the two long-range ordered arrangements indicated in Figure 1⇓. Long-range disorder occurs when …