The word “silt” does not refer to a specific substance. Instead, it is a word used for loose granular particles in a specific size range.

Silt-sized particles range between 0.00015 and 0.0025 inches in diameter, or between 0.0039 and 0.063 millimeters in diameter. They are intermediate in size between coarse clay on the small side and fine sand on the large side.

Grains of coarse silt are large enough that most people can see them without magnification on a background of contrasting color. Most people are not able to sense them if they roll a few grains of silt between their thumb and index finger. Most people are able to detect a few grains of silt by biting them gently between their front teeth. (This test is not recommended, but some experienced geologists and soil scientists use it for quick field identification of silt in sediment and soil.)

Silt does not have a definite composition. It is usually a mixture of clay minerals, micas, feldspars, and quartz. The small-size fraction of silt is mostly clay. The coarse-size fraction is mostly grains of feldspar and quartz.