The fine-grained soil fractions consist of silt and clay; sand and gravel are coarser and not classified as fine-grained.
Soil consists of various sized particles, and geologists often use terms like sand, silt, and clay to describe these. According to the provided information, clay particles are defined as fine-grained, and they possess plasticity, which is attributed to the presence of silt within the clay.
Silt is known to contain minerals such as feldspar and silica, and its particle size ranges between 0.002 and 0.1 mm in diameter, making it also fine-grained. On the other hand, sand is generally coarser than both silt and clay, and thus it is not considered a fine-grained soil fraction.
In conclusion, the fine-grained soil fractions are silt and clay, as opposed to sand and gravel, which are coarser. These fine particles crucially contribute to the plasticity and cohesiveness of the soil, particularly when moisture is present.