The near-infrared spectra of the alkali carbonates.

This study presents the first account of the near-infrared (NIR) spectra of the alkali carbonates: [Cs2CO3] [Rb2CO3] [K2CO3] [Na2CO3] and [Li2CO3]. Seven NIR bands (labelled [A] to [G] inclusive) within the [4000-6000cm-1] (2.5-1.66μm) region of interest are common to the five spectra examined, of which six bands [A-C] and [E-G] proved amenable to quantitative study. The first three occur in the range 4067-4493cm-1 (2.458-2.226μm) and are assigned to a [CO32-] 3v3 overtone, Bands [E] and [F] are centred at ca 4902cm-1 (2.04μm) and ca 5034cm-1 (1.98μm) respectively and are assigned to a (2v1+2v3) combination. Band [G] centred at ca 5190cm-1 (1.92μm) is assigned to a (v1+3v3) combination. One additional band (Band [X]) centred in the vicinity of ca 4080cm-1 (2.45μm) in all spectra other than [Cs2CO3] is assigned to (2v3+2v4). The data is compared with the corresponding additive sum of the mid-infrared (MIR) fundamental, or in the case of combinations, Raman and MIR fundamentals. The quantified differences between NIR band frequency and that of the corresponding MIR derived overtone or combination in the case of [Li2CO3] and [Rb2CO3] closely coincide with Raman active lattice modes of rotary origin. From which it is argued that vibration - libration combinations may operate across a range of NIR frequencies for these mineral types. NIR data from [K2CO3], [Cs2CO3] and [Na2CO3] are discussed in the light of these findings. The influences of differences in atomic mass and space group effects on the NIR spectra of the alkali carbonates are also demonstrated.