Determination of water content in raw perlites: Combination of NIR spectroscopy and thermoanalytical methods

A new approach to determination of water content in raw perlites, an industrially important material, and obsidian was proposed, utilizing diffuse reflectance spectroscopy in the near-IR region. The phase composition of the perlite of the perlite samples was over 94% rhyolitic volcanic glass, with only small admixture of other components. The observed volatile species contents detected from both thermogravimetric analysis (TG) and the loss of ignition method (LOI) varied from 3 to 7%. The samples with the highest content of volatiles released over the temperature interval 30-250 °C (based on thermogravimetric analysis) displayed sharp signals in the ¹H MAS NMR spectra, with chemical shifts of 4.6-4.7 ppm attributed to water molecules of high mobility. Using IR spectra taken in the near-infrared region, the water content of perlites was evaluated using the combination mode (ν + δ)H₂O near 5240 cm^-1. The band area depended on the H₂O content, with the highest value found for the sample which displayed the highest mass loss in the thermoanalytical experiments. The samples showed variations in properties depending on the location in the deposit they were taken from. The relationship between water content determined gravimetrically and calculated band areas showed a correlation coefficient of 0.78 and 0.74 for TG and LOI respectively. The correlation was significantly improved by adding internal standards, hexadecyltrimethylammonium bromide salt (HDTMA) or layered hydrosilicate talc, to the perlite samples, and then normalizing the spectra of the mixtures to selected bands of those standards (R² = 0.89 and 0.88 respectively for TG methods). A better correlation between infrared and TG/LOI results was obtained for HDTMA-Br than for talc. The proposed method using standards could be a reliable way of estimating water content in raw perlites in processing plants.