Lutite is old terminology, which is not widely used, by Earth scientists in field descriptions for fine-grained, sedimentary rocks, which are composed of silt-size sediment, clay-size sediment, or a mixture of both.
Because this is a field term, there is a lack of any precise definition for it based upon specific grain-size characteristics.
Lutites include a variety of fine-grained sedimentary rocks, including calcisiltite, calcilutite, claystone, mudstone, shale, and siltstone.
[1][2] Lutite was first used in 1904 by Grabau,[3] who derived it from lutum, the Latin word for mud.
[1] Pettijohn [5] gives the following descriptive terms based on grain size, avoiding the use of terms such as "clay" or "argillaceous" which carry an implication of chemical composition: