High-pressure behavior of a SiO₂ clathrate observed by using various pressure media

Compression curves and stability of melanophlogite, a natural mineral formed by SiO₂ clathrate, have been studied up to about 25 GPa by using various pressure-transmitting media. We have studied both unheated and preheated samples to understand the role of guest gas molecules in clathrate. All the high-pressure experiments were made at room temperature using a diamond-anvil apparatus combined with synchrotron radiation. Unheated melanophlogite amorphized irreversibly without any structural transition at about 17 GPa, where the volume was decreased to about 70% of its original volume. Identical results were obtained in experiments using methane or an alcohol-water mixture as pressure-transmitting media or when direct compression was applied. Preheated melanophlogite, on the other hand, became much more compressible and amorphized only at around 3 GPa when the volume was decreased to 80%. The behavior changed completely when helium was used as the pressure-transmitting medium. The unheated sample was much less compressible but neither phase transition nor amorphization was observed up to about 25 GPa. Preheated samples had the same compression curve up to about 17 GPa, where an isostructural transition occurred, accompanied by a 10% increase in volume. This isostructural transition was a reversible process with a hysteresis of about 6 GPa. Completely different behaviors observed using helium media suggest that the small helium atom can penetrate into the cage structure of clathrate on compression. Present experimental results will provide useful information to consider the interaction of SiO₂ cages and guest molecules in the clathrate structure.