Introduction

In order to apply crystal field theory to geologic processes involving transition metal ions, it is necessary to have crystal chemical information and thermodynamic data for these cations in mineral structures. In 2.8, it was noted that the principal method for obtaining crystal field splittings, and hence stabilization energies of the cations, is from measurements of absorption spectra of transition metal compounds at wavelengths in the visible and near-infrared regions. The origins of such absorption bands in crystal field spectra were discussed in chapter 3. The focus of this chapter is on measurements of absorption spectra of minerals, with some applications to fundamental crystal chemical problems.

When minerals occur as large, gem-sized crystals, it is comparatively easy to obtain absorption spectra by passing light through natural crystal faces or polished slabs of the mineral. However, most rock-forming minerals are present in assemblages of very small crystals intimately associated with one another, leading to technical problems for measuring spectra of minerals in situ. In addition, several of the transition elements occur in only trace amounts in common minerals, making spectral features of individual cations difficult to resolve, especially in the presence of more abundant elements such as iron, titanium and manganese which also absorb radiation in the visible to nearinfrared region.

Introduction

The crystal chemistry of many transition metal compounds, including several minerals, display unusual periodic features which can be elegantly explained by crystal field theory. These features relate to the sizes of cations, distortions of coordination sites and distributions of transition elements within the crystal structures. This chapter discusses interatomic distances in transition metal-bearing minerals, origins and consequences of distortions of cation coordination sites, and factors influencing site occupancies and cation ordering of transition metals in oxide and silicate structures, which include crystal field stabilization energies

Interatomic distances in transition metal compounds

One property of a transition metal ion that is particularly sensitive to crystal field interactions is the ionic radius and its influence on interatomic distances in a crystal structure. Within a row of elements in the periodic table in which cations possess completely filled or efficiently screened inner orbitals, there should be a decrease of interatomic distances with increasing atomic number for cations possessing the same valence. The ionic radii of trivalent cations of the lanthanide series for example, plotted in fig. 6.1, show a relatively smooth contraction from lanthanum to lutecium.

When the first edition of Mineralogical Applications of Crystal Field Theory was written during 1968–9, it broke new ground by describing results and suggesting applications of the limited spectroscopic and crystal chemical data then available for transition metal-bearing minerals. The data were derived mainly from visible to near-infrared spectral measurements, together with newly available Mössbauer-effect studies of iron minerals, made principally at ambient temperatures and pressures. The book stimulated considerable interest among subsequent mineral spectroscopists who have developed new and improved methods to study minerals and synthetic analogues under a variety of experimental conditions, including in situ measurements made at elevated temperatures and pressures. As a result, the quantity of spectral and crystal chemical data has increased appreciably and may now be applied to a diversity of current new problems involving transition elements in the earth and planetary sciences.

The second edition now attempts to review the vast data-base of visible to near-infrared spectroscopic measurements of minerals containing cations of the first-series transition elements that has appeared during the past 20 years. Several newer applications of the spectral and crystal chemical data are described, including interpretations of remote-sensed reflectance spectra used to identify transition metal-bearing minerals on surfaces of planets. This topic alone warrants the inclusion of a new chapter in the second edition. Many of the classical applications of crystal field theory outlined in the first edition are retained, and each of the original 10 chapters is expanded to accommodate fresh interpretations and new applications of crystal field theory to transition metal geochemistry.

Introduction

In earlier chapters, allusions were made to the effects of covalent bonding. For example, covalent interactions were invoked to account for the intensification of absorption bands in crystal field spectra when transition metal ions occupy tetrahedral sites (3.7.1); patterns of cation ordering for some transition metal ions in silicate crystal structures imply that covalency influences the intracrystalline (or intersite) partitioning of these cations (6.8.4); and, the apparent failure of the Goldschmidt Rules to accurately predict the fractionation of transition elements during magmatic crystallization was attributed to covalent bonding characteristics of these cations (8.3.2).

A fundamental assumption underlying the crystal field model of chemical bonding is that ligands may be treated as point negative charges with no overlap of metal and ligand orbitals. Thus, 3d electrons are assumed to remain entirely on the transition metal ion with no delocalization into ligand orbitals. This situation is never realized, even in ionic structures such as periclase (MgO) and forsterite (Mg2SiO4), let alone bunsenite (NiO), liebenbergite (Ni2SiO4) or fayalite (Fe2SiO4), in which metal–oxygen bonds have some degree of covalent character and electrons in metal orbitals participate in the bonding. Some of the fundamental features of crystal field theory are contrary to expectation or are impossible to derive using the point charge model (Cotton, 1964).

Introduction

One of the most successful applications of crystal field theory to transition metal chemistry, and the one that heralded the re-discovery of the theory by Orgel in 1952, has been the rationalization of observed thermodynamic properties of transition metal ions. Examples include explanations of trends in heats of hydration and lattice energies of transition metal compounds. These and other thermodynamic properties which are influenced by crystal field stabilization energies, including ideal solid-solution behaviour and distribution coefficients of transition metals between coexisting phases, are described in this chapter.

Influence of CFSE on thermodynamic data

Graphical correlations

Crystal field stabilization energies derived spectroscopically from absorption bands in the visible to near-infrared region, including the crystal field spectral measurements of minerals described in chapter 5, are enthalpy terms and, as such, might be expected to contribute to bulk properties such as lattice energies and solvation energies of transition metal compounds. If cations were spherically symmetrical and no preferential filling of 3d orbitals occurred, a given thermodynamic quantity would be expected to display smooth periodic variation in a series of transition metal compounds as a result of contraction of the cations.

Introduction

Considerable interest centres on the Mantle constituting, as it does, more than half of the Earth by volume and by weight. Attention has been focussed on several problems, including the chemical composition, mineralogy, phase transitions and element partitioning in the Mantle, and the geophysical properties of seismicity, heat transfer by radiation, electrical conductivity and magnetism in the Earth. Many of these properties of the Earth's interior are influenced by the electronic structures of transition metal ions in Mantle minerals at elevated temperatures and pressures. Such effects are amenable to interpretation by crystal field theory based on optical spectral data for minerals measured at elevated temperatures and pressures.

In the Mantle, temperatures range up to several thousands of degrees Kelvin and pressures may exceed 100 GPa in the deep interior, attaining 136 GPa at the Core–Mantle boundary. In the past two decades, the optical spectra of several minerals and synthetic analogues have been measured at high pressures and elevated temperatures simulating conditions in the interior of the Earth. The results of many of these high P and T spectral measurements are reviewed in this chapter and applications of the spectral data are described to transition metal-bearing Mantle minerals.

Introduction

In the previous chapter it was shown how measurements of polarized absorption spectra in the visible to near-infrared region can provide information on such crystal chemical problems as oxidation states of transition metal ions, coordination site symmetries and distortions, cation ordering and the origins of colour and pleochroism of minerals. Much attention was focused in chapter 4 on energies of intervalence charge transfer transitions appearing in electronic absorption spectra of mixed-valence minerals.

Perhaps a more fundamental application of crystal field spectral measurements, and the one that heralded the re-discovery of crystal field theory by Orgel in 1952, is the evaluation of thermodynamic data for transition metal ions in minerals. Energy separations between the 3d orbital energy levels may be deduced from the positions of crystal field bands in an optical spectrum, making it potentially possible to estimate relative crystal field stabilization energies (CFSE's) of the cations in each coordination site of a mineral structure. These data, once obtained, form the basis for discussions of thermodynamic properties of minerals and interpretations of transition metal geochemistry described in later chapters.

Introduction

Crystal field theory describes the origins and consequences of interactions of the surroundings on the orbital energy levels of a transition metal ion. These interactions are electrostatic fields originating from the negatively charged anions or dipolar groups, which are collectively termed ligands and are treated as point negative charges situated on a lattice about the transition metal ion. This is a gross simplification, of course, because sizes of anions or ligands such as O2−, OH−, H2O, SO42−, etc., are much larger than corresponding ionic radii of cations (Appendix 3). Two effects of the crystalline field are the symmetry and the intensity of the electrostatic field produced by the ligands. The changes induced on the central transition metal ion depend on the type, positions and symmetry of the surrounding ligands.

Orbitals

The position and energy of each electron surrounding the nucleus of an atom are described by a wave function, which represents a solution to the Schrödinger wave equation. These wave functions express the spatial distribution of electron density about the nucleus, and are thus related to the probability of finding the electron at a particular point at an instant of time.