The size of the particle that constitute soils may vary from that boulders to that of large molecules. Grains larger than approximately 0.06 mm can be inspected with the naked eye or by means of a hand lens. They constitute the very coarse and coarse fractions of the soils. Grains ranging in size from about 0.06 mm to 2 µ (1 µ = 1 micron = 0.001 mm = 1×10−6 m) can be examined only under the microscope. They represent the fine fraction.
Grain smaller than 2 µ constitute the very fine fraction (clay size fraction, CF). Grains having a size between 2 µ and about 0.1 µ can be differentiated under the microscope, but their shape can not be discerned. The shape of grains smaller than about 1 µ can be determined by means of an electron microscope. Their molecular structure can be investigated by means of X-ray analysis.
The process of separating a soil aggregate into fractions, each consisting of grains within a different size range, is known as mechanical analysis. By means of mechanical analysis, it has been found the most natural soils contain grains representative of two or more soil fractions. The general character of mixed-grained soils is determined almost entirely by the character of the smallest soil constituents.
Very coarse fractions, for example gravel, consist of rock fragments each composed of one or more minerals. The fragments may be angular, subangular, rounded, or flat. They may be fresh, or they may show signs of considerable weathering. They may be resistant or crumbly.
Coarse fractions, exemplified by sand, are made up of grains usually composed chiefly of quartz. The individual grains may be angular, subangular, or rounded. Some sands contain a fairly high percentage of mica flakes that make them very elastic or springy. In the fine and very fine fractions, any one grain usually consists of only one mineral. The particles may be angular, flake-shaped, or tubular. Rounded particles, however, are conspicuously absent. If the size of most of the grains in an aggregate of soil particles is within the limits given for any one of the soil fractions, the aggregate is called a uniform soil. Uniform very coarse or coarse soils are common, but uniform very fine or colloidal soils are very seldom encountered. All clays contain fine, very fine, and colloidal constituents, and some clays contain even coarse particles. The finest grain-size fractions of clays consist principally of flake-shaped particles.
The widespread prevalence of flake-shaped particles in the very fine fractions of natural soils is a consequence of the geological processes of soil formation. Most soils originate in the chemical weathering of rocks. The rock themselves consist partly of chemically very stable and partly of less stable minerals. Chemical weathering transforms the less stable minerals into a friable mass of very small particles of secondary minerals that commonly have a scale-like or flaky crystal form, whereas the stable minerals remain practically unaltered. Thus the process of chemical weathering reduces the rock to an aggregate consisting of fragments of unaltered or almost unaltered minerals embedded in a matrix composed chiefly of discrete scaly particles. During subsequent transportation by running water the aggregate is broken up, and the constituents are subjected to impact and grinding. The purely mechanical process of grinding does not break up the hard equidimensional grains of unaltered minerals into fragments smaller than about 10 µ (0.01 mm). On the other hand, the friable flake-shaped particles of secondary minerals, although initially very small, are readily ground and broken into still smaller particles. Hence, the very fine fractions of natural soils consist principally of flake-shaped particles of secondary minerals.
Source: Soil Mechanics in Engineering Practice by Terzaghi et al