The USC (unified soil classification), which is mostly as adpoted by ASTM includes four principal fine-grained soils: Silt, Elastic Silt, Lean Clay and Fat Clay. There is a fifth soil class called, "Silty Clay," but it's very rare (notwithstanding the use shown above).
(Just to avoid confusion, if you come from the USDA tradition of soil judging where soils are classfied by weight percent, then silty clay has a different meaning, which I'm not going to further reference.)
The USC and the ASTM soil classification methods define "fine-grained" soils (i.e., silt and clay soils) as those soils with greater than 50 percent of the soil grains finer than the No. 200 sieve (i.e., 0.074 mm). Anything coarser than 0.074 mm is a sand (well, or a gravel).
The USC and ASTM soil classification methods use behavoir to classify silt and clay. There is no consideration for grain size in classifying silt or clay. Rather, silt and lean clay have liquid limits less than 50 and elastic silt and fat clay have liquid limits greater than 50. The distinction between silt and lean clay is determined by the "A" line. If the plasticy index is greater than the value 0.73(LL-20) than it's a lean clay if it's less than it's a silt. The distinction between elastic silt and fat clay is also determined by the "A" line and if the PI is greater than 0.73(LL-20) it's a fat clay and if it's less it's an elastic silt. So, there is not that much real estate between a silt with a LL of 49 and a PI of 21 and a fat clay with a LL of 51 and a PI or 24.
Confused? A picture is worth a lot of words, eh? Please note the small area for CL-ML soils (i.e., clayey silt) - a widely misclassified soil type!
f-d
¡papá gordo ain’t no madre flaca!
