Short-range atomic arrangements in minerals. I: The minerals of the amphibole, tourmaline and pyroxene supergroups

Abstract

Spectroscopy is a key aspect of deriving local arrangements of atoms in minerals. Vibrational spectroscopy in the principal O–H-stretching region and MAS NMR spectroscopy are sensitive to such local arrangements, and play a crucial role in characterizing local structure in minerals in which there is solid solution. Variation in local arrangements of ions around a “probe” ion such as (OH)− can produce shifts in the energy of the principal O–H-stretching frequency, providing a window into those local arrangements. Similarly, variation in local arrangements around a “probe” isotope such as 27Al or 29Si can produce shifts in resonance energy that are indicative of differences in local environment. It is useful to develop a configuration symbol for the structural environment of the probe species, i.e. , the configuration of nearest-neighbour and next-nearest-neighbour polyhedra/sites, and then the local arrangements of atoms may be expressed in terms of this configuration symbol. Work on the monoclinic C 2/ m amphiboles, tourmaline and monoclinic pyroxene is reviewed here in terms of the particular effects that can give rise to absorptions in the principal O–H-stretching region of vibrational spectra. It is notable that the nearest-neighbour configurations in the amphibole, tourmaline and mica structures are topologically identical, and hence there should be strong spectral characteristics that are common to minerals of all three structure types. The spectra of the C 2/ m amphiboles show strong next-nearest-neighbour effects, and one expects such effects to occur also in spectra of minerals of the tourmaline (and mica) supergroups. The valence-sum rule of local bond-valence theory provides a strong constraint on possible local arrangements involved in heterovalent solid-solution in these minerals.