How big is Clay Particle? | Scale of the Universe

A clay particle is one of the finest grains found in soil and sediment. Soil scientists call any mineral grain smaller than 2 micrometers across clay. The 2 micrometers shown here is that upper limit, the largest a clay particle can be, not the size of a typical one. A micrometer is a millionth of a meter.

Size

Soil scientists sort the mineral grains in soil into classes by size. In the system used by the Food and Agriculture Organization of the United Nations, grains smaller than 2 micrometers are clay. Grains from 2 to 63 micrometers are silt, and grains from 0.063 to 2 millimeters are sand. Many countries use the same names, but some draw the boundaries in different places.

Geologists who study sediments often use a different scale, called the Wentworth scale. On it, clay is anything smaller than 4 micrometers, silt runs from 4 to 62.5 micrometers, and sand from 62.5 micrometers to 2 millimeters. So whether a grain 3 micrometers across counts as clay depends on which scale is used.

The word clay also names a family of minerals that are built up in thin layers. Grains of clay minerals range from a few nanometers, or billionths of a meter, to millimeters across. So a clay-sized grain is not always a clay mineral: it can also be a tiny crystal of another mineral, such as quartz.

Size comparisons

The largest grains of sand, 2 millimeters across, are 1,000 times as wide as the largest clay particles.
If a clay particle 2 micrometers across were enlarged to the size of a tennis ball, about 6.7 centimeters (2.6 inches) across, the smallest grains of sand would be about 2 meters (7 feet) across. The largest would be about 67 meters (220 feet) across.
About 40–50 of the largest clay particles, side by side, would span the width of a human hair, which is 80–100 micrometers wide.
Particles of the giant virus Pithovirus sibericum are about 1.5 micrometers long, smaller than the largest clay particles.

How clay feels

Soil scientists working outdoors can estimate how much clay a soil holds by rubbing a moist sample between their fingers. Clay is sticky, can be molded into shapes, and looks shiny after it is squeezed. Silt is not sticky and feels floury, like talcum powder. Sand feels grainy and cannot be molded at all.

Clay and water

Clay minerals attract water strongly. Some kinds swell when they get wet: a single clay particle 1 nanometer thick can grow to about 2 nanometers thick in water. Houses and other buildings on soils with swelling clays can be damaged as the clay swells and shrinks with the seasons.

Mixed with a little water, clay makes a mud that can be shaped and then dried into a fairly rigid solid. Potters and the ceramics industry use this to make plates, cups, bowls and pipes.