Vibrational spectroscopy of calcic amphiboles – applications for exploration and mining

Calcic amphiboles in form of single crystals and in rock samples have been measured using laboratory-based infrared reflectance spectroscopy (IRS) and routine IRS technologies applied in mineral exploration. The composition of amphiboles in selected rock samples was validated with scanning electron microscopy (SEM) and electron microprobe work. Published values for wavenumber frequencies of hydroxyl-related stretching and bending vibrations were compared with the results from our study and both were combined to calculate combinations and overtones of [M1M1M3]-O-H in the short-wave infrared regions of 5000–4080 cm −1 (2000–2450 nm) and 7350 cm −1 (1360 nm) regions, respectively. Detailed comparison of major short-wave infrared absorption features in various calcic amphiboles and talc leads to the conclusion that an absorption feature centred at 2080 nm is diagnostic for talc and can be used to distinguish amphibole from talc. Multiple feature extraction scripts were developed to determine the relative abundance of amphibole and talc, as well as the Mg# of amphiboles in large IRS data sets. Our results show that only the 2390 nm absorption feature in amphibole can be reliably used to determine its abundance and Mg# in mineral assemblages containing other short-wave infrared active minerals. Different mafic and ultramafic lithologies can be inferred from infrared hyperspectral drill core logging and remote sensing datasets, based on the developed scripts.