Study of the infrared spectra of phyllosilicates through direct measurements, quantum mechanical modeling, and analysis of AVIRIS imaging spectrometer data: Relationships with environment of mineralization

Hydrothermal phyllosilicates have been studied in order to: (1) establish the connections between their infrared spectra and octahedral composition, (2) review and clarify the relationships between octahedral composition and temperature of formation, (3) investigate the utility of quantum mechanical models to calculate infrared frequencies of phyllosilicates, and (4) assess the adequacy of using imaging spectrometer data to detect composition-related spectral variability in these types of minerals.;These questions have been investigated through direct measurements (laboratory, field and imaging spectrometry) and modeling of infrared spectra of ideal minerals using quantum mechanical techniques. Octahedral composition of phyllosilicates may be estimated from spectroscopic studies, since hydroxyl groups present in the octahedral sheet are extremely sensitive to the nature of neighboring octahedral cations.;Infrared spectra of phyllosilicates have been satisfactorily modeled using ab initio quantum mechanical techniques. Not only have known experimental frequencies been reproduced, but frequencies not described in the literature have been predicted. Physical parameters governing the effects of neighboring octahedral cations on the hydroxyls' fundamental vibrations have been proposed. Also, differences between the effect of octahedral cations on OH vibrations in di- and trioctahedral environments have been established.;The findings obtained from laboratory analysis and modeling have been extrapolated to the regional scale, using AVIRIS imaging spectrometer data collected over the district of Yerington, Nevada. AVIRIS data show spectral variability in phyllosilicates comparable to that predicted by modeling and observed in laboratory spectra.;Experimental and theoretical studies in the literature show a close relationship between octahedral composition of phyllosilicates and temperature of mineralization, diagenesis, or metamorphism. These studies indicate that the amount of octahedral Mg and Fe in dioctahedral phyllosilicates decreases with increasing temperature. However, other factors such as bulk rock and fluid composition, and kinetic history, may influence the octahedral composition of phyllosilicates. Compositional variations detected in AVIRIS data of Yerington and confirmed by geochemical analysis of phyllosilicate samples, can be explained by gradual changes in the bulk composition of the magma that gave rise to the tuff deposits hosting these phyllosilicates.