A claystone is a lithified and non-cleavable mudrock. In order for a rock to be considered a claystone, it must consist of at least fifty percent clay (phyllosilicates), whose particle measures less than 1/256 of a millimeter in size. Clay minerals are integral to mudrocks, and represent the first or second most abundant constituent by volume. They make muds cohesive and plastic, or able to flow. Clay minerals are usually very finely grained and represent the smallest particles recognized in mudrocks. However, quartz, feldspar, iron oxides, and carbonates can also weather to the sizes of typical clay mineral grains.