Field studies of air bubble and clathrate Inclusions in the NGRIP ice core
reveal two stages of clathrate formation. In the transition zone translucen
t primary clathrates with rough surfaces and irregular shapes, formed from
individual bubbles, split up and form smaller specimens. Deeper down in the
transition zone (similar to 1100 m), clathrate metamorphosis sets in, givi
ng rise to the formation of transparent secondary clathrates, with smooth a
nd regular, often polyhedral or faceted shapes. Rod-like crystals of great
length (similar to 2 mm) often show signs of separation into smaller units
that fragment into smaller specimens. These findings imply that the simplif
ied picture of one bubble forming one clathrate has to be modified.