Weathering detected by Raman spectroscopy using Al-ordering in albite

Abstract

Albite samples from Ontario, Canada, laboratory weathered in HCl solutions at pH 1 have been analysed and compared to unreacted mineral samples with the same crystal orientation using Raman microspectrometry. The study was focused on Raman bands reflecting Al-ordering because destabilisation of Al–O bonds with subsequent release of Al is well recognised to be a major factor in low pH dissolution of albite. Analyses of unweathered samples reveal that most bands in the albite spectra show variations in intensities for different crystal orientations in the wavenumber range 100 to 1200 cm −1. A well-defined crystal orientation is therefore essential to a successful investigation of weathered samples. Unweathered albite has two bands; 455 and 976 cm −1 in the spectrum normal to the (001) face, which have been related to Al-ordering in the structure. In spectra of weathered albite crystals, with the same orientation, the band most sensitive to alteration is 976 cm −1. This band is very strong for unreacted albite, but diminishes in intensity relative to the other bands at the initial stage of weathering. When dissolution continued, the structural information in the range 700 to 1200 cm −1 was completely lost, shown as a bandbroadening of a less organised structure, while the characteristic albite bands still could be observed in the range 100 to 600 cm −1.