Negative octahedral snowflakes or Tyndall figures in tetrahydrofuran clathrate hydrate

Abstract

When a piece of ice Ih whose grains are not too small is irradiated by infrared or radio-frequency light, internal melt figures are often formed. They are often called Tyndall figures or Tyndall flowers after John Tyndall, who was the first to understand them¹. No other substance has been reported to form internal melt figures, although any substance that melts to a denser liquid should form them at the proper conditions. The shape of the figures will, of course, depend on the symmetry of the crystal. Tetrahydrofuran clathrate hydrate², which has the nominal composition C₄H₈O.17H₂O, melts to a denser homogeneous liquid at 4.1 °C with a fractional change of volume of −0.0147±0.0003³. Internal melt figures should therefore form inside the clathrate if it is heated internally. From the symmetry and the structure of the crystal, the figures should be octahedra². This letter describes this second type of internal melt figure, which was discovered 130 years after the first, and suggests that melt figures should occur in many other crystals.