The term calcilutite was originally proposed in 1903 by Grabau[2][3] as a part of his calcilutite, calcarenite and calcirudite classification system based upon the size of the detrital grains composing a limestone.
[4][5] In the original classification[2][3] of limestone according to the dominant grain-size, calcisiltites were not named and are classified as calcilutite.
In this classification, which the majority of geologists follow, a calcilutite consists of both silt- and clay-size, less than 0.062 mm in diameter, grains.
It is the carbonate equivalent of a mudstone (not to be confused with a 'mudstone' of the Dunham Limestone classification).
[5][6] Calcilutites can accumulate in a wide variety of marine and lacustrine environments.