Ar,N2, and CO2 in the structural cavities of cordierite,an optical and X-ray single-crystal study

Ar, N₂ and CO₂ were introduced into the structural cavities of channel-evacuated single-crystals of White Well cordierite with the composition: K_0.01Na_0.03(Mg_1.91Fe_0.09Mn_0.01)Al_3.98Si_5.01O₁₈. The gas refilling experiments were carried out in conventional hydrothermal bombs at 6–7 kbar and 600–700°C. The increase in the mean refractive indices for gas-treated crystals, as determined with a spindle-stage equipped microscope, was used along with point-dipole calculations to estimate the percentage of occupied structural cavities. The steep increase of the electronic polarizability parallel to the a-axis, which can be derived from the increase of the refractive index n _γ (Z∥a) upon introduction of volatiles, indicates that N₂ and CO₂ are preferentially aligned parallel to the a-axis of cordierite. Single-crystal structure refinements at room temperature confirm these predictions. Additionally, decreased C–O and N–N bond lengths suggest a librational motion with a mean rotary oscillation angle of 35° (N₂) and 25° (CO₂) about a, where c is the rotation axis. Mean libration angles of 40° (N₂) and 28° (CO₂) were estimated from the electronic polarizability tensors of CO₂ and N₂. Site occupancy refinements of the channel position are in good agreement with the optically derived values for the volatile concentrations, both indicating about 70% and 60% filled cavities for Ar- and N₂-cordierite, respectively. Chemical analyses and point-dipole calculations confirm that about 45% of the cavities are occupied in the CO₂-treated crystal. The structural framework of cordierite is slightly but specifically altered by the various channel occupants.