Luminescence of Kaolinite

-Dehydration-induced luminescence (DIL), the emission of light from a clay paste upon dehydration, was characterized experimentally for a colloidal kaolinite. The relationship between total photon count of the emitted light and film thickness is linear up to a thickness of 30/~m. The photon emission was obtained over a critical range of water contents (25-60%) of the oven-dry clay, and the kinetics of photon emission was presumed to be closely associated with the kinetics of film dehydration. Whether drying proceeded throughout the bulk or via a moving front was undetermined, but in either mode it was preceded by the formation of a thin dry film at the interface with the atmosphere. Grinding of the kaolinite for several minutes by mortar and pestle before paste preparation resulted in an overall increase of photon emission compared to unground kaolinite and in the formation of more than one emission peak, as well as a prolongation of the light emission. This effect on the kinetics of light emittance was observed for about two months after the application of the mechanical stress and suggests a means of detecting the mechanical stress history of a clay. An estimate was made of the spectral characteristics of the emitted light using optical filters and by incorporating tryptophan and salicylic acid into the kaolinite paste where they acted as fluorescent probes. The latter technique shifted the frequency of the light emitted by the kaolinite from the ultraviolet to the visible range where it was less effectively reabsorbed. The first method showed that the wavelengths of 97% of the emitted light was <460 nm and that 75% of the light had wavelengths <410 nm. The second method showed that the total intensity of DIL increased in the presence of fluorescence molecules, suggesting that the emittance was in the ultraviolet range. Key Words--Dehydration, Fluorescent probes, Grinding, Kaolinite, Luminescence. I N T R O D U C T I O N Dehydration-induced luminescence (DIL) is the emission of light from a clay paste upon dehydration (Coyne et al., 1980, 1981). It is typified by an initial low photon-emission rate which decays monotonically, followed by a sharp rise to a peak emittance, and then by a decay to the background level, the rise and fall occurring over a time scale of approximately 30 rain. Under certain conditions (to be discussed below), the luminescence is typified by more than one peak and by a long tailing off of light emission. Dehydration-induced luminescence is one of several novel luminescence effects first reported by the authors (Coyne et al., 1980, 1981, 1983, 1984; Lahav et al., 1982). The only other luminescence from kaolin to have been reported is thermoluminescence, first described by Seigel et al. in 1966. Thermoluminescence of several structurally related materials was reported by Ferraresso in 1967, greatly elaborated on by Nishita and Hamil ton (1968, 1970, 1971, 1972, 1973, 1975) and Nishita et al. (1975), and others and recently reinvestigated by Lemons and McAtee (1982). The atomic/molecular origins of the various luminescence effects have not yet been definitively assigned, 1 Hebrew University of Jerusalem. 2 San Jose State University, San Jose, California. Copyright 9 1985, The Clay Minerals Society although they have been tentatively associated with trapped electronic excitations centered near structural cation-substituted sites. Also, the mechanisms of radiative excitation and relaxation have not been completely enumerated or characterized for clay minerals, but it is thought that mechanical stress and/or interfacial effects on the energy level diagram may play a role in conjunction with, or independent of, the more traditionally studied gamma irradiation. To investigate this phenomenon and to observe the effects of different clay treatments on DIL, the phenomenon must be reproducible with a reasonable degree of accuracy. The purposes of the present work are to characterize further and to standardize the measurement of the DIL of a colloidal kaolinite, and to determine the effect of various treatments on the luminescence characteristics. E X P E R I M E N T A L Colloidal kaolin, lot 753828, from the Fisher Scientific Company was used without further purification. The surface area was measured by B.E.T. by Pacific Sorption Service, Chico, California, to be 15 mE/g. The hygroscopic water content is 3-4% and the cation-exchange capacity of Fisher material is 1.03 meq/100 g, as reported by White and Erickson (1980). The trace metal composit ion of this material was reported in Coyne et al. (1983). L-tryptophan A-grade was ob-