Infrared spectra and crystal chemistry of scapolites: implications for Martian mineralogy
Near‐infrared and midinfrared spectra of a wide range of scapolite compositions were studied to determine the cause of the 2.36‐μm features that have been correlated with similar features in the near‐IR spectrum of Mars. We attribute the 2.36−μm features to vibrations caused by HCO3− and HSO4− in the anion sites of scapolite. The 2.36‐μm absorption complex consists of four overlapping bands with the following assignments: absorptions at 2.33 and 2.35 μm are attributed to combinations of hydroxyl stretches and a O‐H … O bend from HCO3−, absorptions at 2.39 and 2.41 μm are attributed to combinations of hydroxyl stretches and a O‐H ··· O bend from HSO4−. The relative intensities of all four bands vary according to the HCO3−/HSO4− ratio and disordered anion site occupancy. Oriented single‐crystal polarized spectra of a bicarbonate‐bisulfate bearing scapolite show that the 3.9‐μm hydrogen bond stretch, from HCO3−, is polarization controlled. This verifies that HCO3− is an intimate part of the scapolite crystal structure and that hydrogen in HCO3− is forming hydrogen bonds with T1 framework oxygens. The association of yellow fluorescence, attributed to S2− in the anion site, with vibrational features of HCO3− and HSO4− in the rims and fractures of scapolite grains suggests that some common geochemical process is incorporating these ions into the crystal structure during later stage alteration of scapolite. The positional disorder of HCO3− and HSO4− in the low‐symmetry anion site of scapolite gives the 2.36‐μm band complex a unique spectral signature not likely to be duplicated in any other mineral.