A research submarine was used to sample an amber-colored gas hydrate e
xposed on the sea-floor at 540 m water depth in the Gulf of Mexico con
tinental slope, offshore Louisiana. The hydrate composition is novel f
or a natural occurrence because i-C-5 comprises 41.1% of the total C-i
-C-5 hydrocarbon distribution. The relative abundance of i-C-5 is cons
istent with an hexagonal (H) lattice structure that is capable of encl
osing larger hydrocarbon molecules than either structure I or II hydra
tes. Structure H hydrate, which has not been identified previously in
nature, coexists with structure II hydrate on the Gulf slope.