Refractive-index measurements of natural air–hydrate crystals in an Antarctic ice sheet

The refractive index of air–hydrate crystals found in a deep Antarctic ice sheet was measured for the first time, as far as we know, using a Mach–Zehnder interferometer. A small difference between the refractive indices of the air–hydrate crystals and the matrix ice crystal was measured by the fringe-shift method. It was found that the refractive indices of all air–hydrate crystals were larger than those of ice, and the average difference was 5.3 × 10−3, even considering the refractive-index anisotropy of ice crystals. Because the refractive indices depend on the occupancy ratio of cagelike cavities by air molecules, we compared the experimental results with the calculated values using the Onsager cavity model. We determined that the present method is useful for estimation of the cavity occupancy ratio of air–hydrate crystals and also of the amount of air molecules in polar ice cores.