A mudstone is a siliciclastic sedimentary rock which has a mixture of silt- and clay-sized particles (at least 1/3 of each). So, a mudstone is a type of mudrock with a more varied grain size than either siltstones or claystones.
A claystone is a mudrock which does not split easily.
In order for a rock to be a claystone, it must be up to half (50%) clay, whose particles measure less than 1/256th of a millimeter.
- Blatt H; Middleton G & Murray R. 1980. Origin of sedimentary rocks. 2nd ed, Prentice Hall, Englewood Cliffs, N.J. ISBN 0-13-642702-2
- Kearey P. The new Penguin dictionary of geology. London: Penguin Books. p176