Quantitative Infrared Spectra of Hydrosilicates and Related Minerals

Absorption coefficients associated with atomic motions of species expected in astronomical environments are determined from infrared measurements of various hydrosilicates, hydrated magnesium oxide, and the Al-bearing chain silicate, sapphirine. Band types measured include O—H stretching modes near 3 μm, Si—O stretching motions near 10 μm, Si—O—Si bends near 14 μm, O—Si—O bends near 20 μm, and translations of cations such as Mg and Ca near 50-200 μm. We obtain data from films of varying thickness and use a ratioing method. First, bandstrengths of O—H fundamentals were determined from spectra obtained from films of controlled thicknesses, generally 6 μm. The O—H absorbance strength was then used to accurately determine thickness for a thinner film of each mineral (found to be <1 μm), thus providing bandstrengths of all other absorptions. Thin films were prepared such that the fundamental lattice modes showed intrinsic behaviour (i.e. band shapes were unchanged upon further thinning) and O—H modes are well resolved above the spectral noise. Bandstrengths were found to depend weakly on structure and should be applicable to other silicate minerals, allowing estimation of elemental concentrations independent of knowing the speciation of dust in astronomical environments. Comparison with observational data of NGC 6302 suggests that lizardite and saponite could be present in addition to refractory minerals.