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.