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. When mixed with water lutites often disintegrate into mud. 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. It is equivalent to the term mudstone and the Greek-derived term pelite.[1] [2] Lutite was first used in 1904 by Grabau,[3] who derived it from lutum, the Latin word for mud. He also proposed a number of prefixes to be used with and attached to "lutite" in order to designate various types of lutites. None of these prefixes are used by Earth scientists nowadays.[1]
Pettijohn [4] 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:
| Texture | Common | Greek | Latin | |
|---|---|---|---|---|
| Coarse | gravel(ly) | psephite (psephitic) | rudite (rudaceous) | |
| Medium | sand(y) | psammite (psammitic) | arenite (arenaceous) | |
| Fine | clay(ey) | pelite (pelitic) | lutite (lutaceous) |