Optical Expressions of Ion-Pair Interactions in Minerals

Citation

Mattson, Stephanie Margaret (1985) Optical Expressions of Ion-Pair Interactions in Minerals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/acn3-nr54. https://resolver.caltech.edu/CaltechETD:etd-04082005-133814

Abstract

Clusters of transition element cations in neighboring sites frequently govern the optical properties of minerals. This is particularly true of Fe-bearing minerals which may exhibit several types of ion-pair transitions. In this thesis four different types of interactions were distinguished: intensified spin-forbidden transitions of Fe³⁺ clusters, intensified Fe²⁺ spin-allowed transitions of Fe²⁺-Fe³⁺ clusters, heteronuclear charge transfer transitions of Fe²⁺-Ti⁴⁺ and Mn²⁺-Ti⁴⁺ clusters, and homonuclear charge transfer transitions of Fe²⁺-Fe³⁺ clusters.

The optical characteristics of Fe³⁺ in red Fe³⁺ rich and black Fe³⁺,Fe²⁺-rich tourmalines were examined by absorption spectroscopy in the visible and near-infrared, Mössbauer spectroscopy and magnetic susceptibility measurements. Intense optical absorption features at 485 and 540 nm were assigned to transitions of exchange-couple Fe³⁺ pairs in two different site combinations. Absorption spectra at variable temperatures and of samples which were oxidized and reduced were used to establish these assignments. Site assignments were based on intensity ratios in different polarizations according to the polarization of these transitions along the vector between the interacting cations. The 485 nm band occurs at an unusually low energy for Fe³⁺ in silicate minerals. Similar behavior was observed in the spectrum of coalingite, a Mg,Fe-hydroxy carbonate, and has been proposed to result from magnetic exchange in large, edge-shared octahedra. The antiferromagnetic exchange which is generally associated with intensity increases in Fe³⁺ clusters was confirmed by variable temperature magnetic susceptibility measurements. The Mössbauer spectrum of a red tourmaline with 3.4% Fe exhibits an unusual decrease in width of peaks by ~30% from 298 K to 5 K which may be related to an unusual interaction between Fe³⁺ and trace amounts of Fe²⁺.

Optical absorption and Mössbauer studies of Fe²⁺-bearing tourmalines with variable Fe³⁺ contents were used to examine Fe²⁺ transitions which are intensified through an interaction with Fe³⁺ neighbors. The variation of molar absorptivity of Fe²⁺ bands with the fraction of Fe²⁺ in Fe²⁺-Fe³⁺ pairs indicates that Fe³⁺ ions increase the absorptivity of Fe²⁺ bands to ~1200 M^-1cm^-1 as compared to ~5 M^-1cm^-1 for non-interacting Fe²⁺. Approximately equal degrees of intensification were observed for both components of the ⁵T₂ → ⁵E Fe²⁺ transition as well as for Fe²⁺ in two different sites. Although the detailed behavior of non-interacting Fe²⁺ ions differ in Mg-tourmalines and Li,Al-tourmalines, the characteristics of Fe²⁺-Fe³⁺ absorption are constant. Intensity increases were restricted to the polarization which coincided with the vector between the Fe²⁺ and Fe³⁺ ions. The intensified Fe²⁺ transitions are characterized by an unusual temperature response. The integrated intensity of these transitions increases by 10-50% at 83 K as compared to 296 K. The positions and widths of the intensified transitions maintain the values of the non-interacting Fe²⁺. Tourmalines with the lowest Fe³⁺ contents were the gemmy Li,Al-tourmalines which generally form in pockets within pegmatites. Fe,Mg-tourmalines exhibited consistently higher Fe³⁺ contents than any of the Li-bearing tourmalines examined. Oxidation of Fe²⁺ which resulted from gamma irradiation of blue Li-tourmalines which contained several percent each of MnO and FeO could be monitored by increases in Fe²⁺ intensity in one polarization.

Fe²⁺-Ti⁴⁺ charge transfer transitions were examined in minerals which contain stoichiometric quantities of Fe and Ti -- taramellite, neptunite, and traskite -- and tourmaline. The wavelength of these transitions ranged between 400 and 500 nm, and the halfwidths ranged between 7000 and 9000 cm^-1. These characteristics can generally be used to assign Fe²⁺-Ti⁴⁺ charge transfer transitions. The molar absorpitivities of these transitions, however, exhibit very large variations. The molar absorptivity of Fe²⁺-Ti⁴⁺ charge transfer in neptunite is ~225 M^-1cm^-1 in beta polarization, in taramellite it is ~1300 M^-1cm^-1 and in tourmaline it is ~4000 M^-1cm^-1. Tentative assignments of Fe²⁺-Ti⁴⁺ in more dilute minerals generally compare favorably with the energy and width stated above. However, sapphire and other Al-minerals such as kyanite have very different characteristics for bands assigned to Fe²⁺-Ti⁴⁺ charge transfer. The Fe²⁺-Ti⁴⁺ charge transfer transition in taramellite exhibits no change in integrated intensity with decreasing temperature but increases by 15% from 296 K to 83 K in tourmaline. Mn²⁺-Ti⁴⁺ charge transfer was also assigned to a transition at 320 nm in two unusual yellow tourmalines.

The characteristics of Fe²⁺-Fe³⁺ charge transfer transitions were reviewed in light of recent data and with regard to their utility as diagnostic criteria. A correlation between charge transfer energy and the separation of the interacting cations proposed by Smith and Strens (1976) could not be supported by the expanded data base. Temperature variations of charge transfer transition areas were also examined. The magnetic behavior of two minerals which exhibited different temperature responses were investigated. General agreement with the theories of Cox (1980) and Girered (1983) that suggest that ferromagnetic exchange should produce intensity increases at low temperature and that antiferromagnetic exchange produces intensity decreases was confirmed by these examples of Fe²⁺-Fe³⁺ charge transfer but could not explain the temperature response of Fe²⁺-Ti⁴⁺ charge transfer transitions. In any case, an increase in intensity with decreasing temperature, which is generally expected on the basis of experimental observations, cannot be used in a negative sense to eliminate a charge transfer assignment. The large width of charge transfer transitions is generally the most useful diagnostic criterion.

Cox, PA (1980) Electron transfer between exchange-coupled ions in a mixed-valency compound. Chem Phys Lett 69: 340-343

Girerd, J-J (1983) Electron transfer between magnetic ions in mixed valence binuclear systems. J Chem Phys 79: 1766-1775

Smith, G and Strens, RGJ (1976) Intervalence transfer absorption in some silicate, oxide and phosphate minerals. In: The Phyics and Chemistry of Minerals and Rocks, Strens, RGJ (ed.). New York: Wiley and Sons, pp. 583-612

Thesis Files