A metamorphic rock, intermediate between shale and slate, that does not possess true slaty cleavage.
A hardened mudstone showing no slatelike cleavage.
A highly compacted sedimentary or slightly metamorphic rock consisting primarily of particles of clay or silt. Argillite differs from mudstone in that it does not have the same fine laminations, and from shale and slate in that it is not fissile.
Origin of argillite
- Latin argilla argil argil –ite
From American Heritage Dictionary of the English Language, 5th Edition