How can we differentiate clay and silt apart from its sizes?

Here, from a geotechnical perspective you can’t differentiate silt and clay on the basis of grain size because it can be irrelevant to material behaviour.

Silt and clay are both the result of the physical and chemical breakdown of the minerals in rocks. They main difference is in chemical composition and particle size.

Silt is composed of silicate minerals, or those containing silicon and oxygen.

Clay is composed of metal silicates, or silicates with metals like magnesium or aluminum associated with it.

But in terms of handling the two as part of an experiment, the main discernible difference is particle size. Sand particles are larger than silt particles which are in turn larger than clay particles. Silt particles are about 0.05 to 0.002 millimeters in size. Clay particles are smaller than micrometer in size. So silt particles are too small to really be felt with your fingers like grains of sand. But wet silt will feel very smooth like finely ground baking flour. Clay particles are so small that they will simply feel sticky. You won’t be able to move them around your fingers like silt particles. This stickiness will be the main difference between the two in terms of touch.

Clay vs silt:

• The super most fine particle is called as clay which has a property to hold water.
• The size of the clay particle is less than 2 Micron
• The size of silt is in between to Micron to 75 Micron.
• The texture of clay and silt is different. (Refer above a figure.)
• When we talk about building construction then black cotton soil is not suitable because it consists of a mineral named montmorillonite which is responsible for high shrinkage and swelling property.
• Silt can be easily compacted. Silt can be carried by water during flood time and it forms of fertile deposit on the surface.

Silt is composed of fine-grained clay in usual parlance. However, note the Wiki comment:

Though silt and clay are classified as fine soils, they have some differences between them. – Clay particles are very smaller in size compared to silt particles, even though all soils with particle size <0.075mm are classified as either silt or clay. – Clay contains clay minerals, while silts do not contain clay minerals.

Ribbon test – clay will form a ribbon if a mixture of water and clay is rolled in hand – clay will join with other particles of clay to form a molecular string fused to each other. Silt will not work for this.

A non-technical test is to see whether the soil is a fertile plant-growth footing. True clay isn’t. Example: kudzu jungles here in Appalachian supposed “clay country.”