1. A rock or mineral fragment or a detrital particle of any composition smaller than a very fine silt grain, having a diameter less than 1/256 mm (4 microns, or 0.00016 in., or 8 phi units.) This size is approximately the upper limit of size of particle that can show colloidal properties. 2. A loose, earthy, extremely fine-grained natural sediment or soft rock composed primarily of clay-size or colloidal particles and characterized by a considerable content of clay minerals and subordinate amounts of finely divided quartz, decomposed feldspar, carbonates, ferruginous matter, and other impurities. It forms a plastic, moldable mass when finely ground and mixed with water, but retains its shape on drying, and becomes firm, rocklike, and permanently hard on heating or firing. 3. A term that is commonly applied to any soft, adhesive, fine-grained deposit (such as loam or siliceous silt) and to earthy material, particularly when wet (such as mud). 4. A term used by the International Society of Soil Science for a rock or mineral particle in the soil, having a diameter less than 0.002 mm (2 microns).