Silt - Clay - Mud - Sand - Soil: What Is The Difference?
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz (made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2) and feldspar ( KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8). Silt may occur as a soil or as suspended sediment (also known as suspended load) in a surface water body. It may also exist as soil deposited at the bottom of a water body. silt particles range between 0.0039 to 0.0625 mm or 3.9 and 62.5 micons.
Clays are formed from thin plate-shaped tiny particles held together by electrostatic forces, so there is a cohesion. According to the USDA Soil Texture Classification system, the sand-silt distinction is made at the 0.05 mm particle size. The USDA system has been adopted by the Food and Agriculture Organization (FAO). In the Unified Soil Classification System (USCS) and the AASHTO (American Association of State Highway and Transportation Officials) Soil Classification system, the sand-silt distinction is made at the 0.075 mm particle size (i.e. material passing the #200 sieve). Silts and clays are distinguished mechanically by their plasticity (the deformation of a material undergoing non-reversible changes of shape in response to applied forces). This property is measured using indexes know as PL and LL:
The Liquid Limit (LL) is the moisture content at which a fine-grained soil no longer flows like a liquid. The Plastic Limit (PL) is the moisture content at which a fine-grained soil can no longer be remolded without cracking.
One famous type of clay is Kaolin: Kaolin, also called china clay, soft white clay that is an essential ingredient in the manufacture of china and porcelain and is widely used in the making of paper, rubber, paint, and many other products. Kaolin is named after the hill in China (Kao-ling) from which it was mined for centuries.
Another important clay is Bentonite. Bentonite is a highly absorbent, viscous plastic clay which is a valuable binding, sealing, absorbing and lubricating agent in a huge variety of industries and applications. Sodium bentonite expands when wet, absorbing as much as several times its dry mass in water. Because of its excellent colloidal properties, it is often used in drilling mud for oil and gas wells and boreholes for geotechnical and environmental investigations. The montmorillonite making up bentonite is an aluminium phyllosilicate mineral whose crystal structure is described as low-charge TOT. This means that a crystal of montmorillonite consists of layers, each of which is made up of two T (tetrahedral) sheets bonded to either side of an O (Octahedral) sheet. Bentonite is also used as cat litter due to its odor adsorbing properties. Bentonite is also used in edible oil extraction as well as an animal feed additive: it improves pellet quality, prevents caking, improves milk production in cows, wool growth in sheep and egg production with chickens.
Fuller's Earth is a term for various clays used as an absorbent, filter, or bleaching agent. Products labeled Fuller's earth typically consist of palygorskite (attapulgite) or bentonite. Primary modern uses include as absorbents for oil, grease, and animal waste (cat litter), and as a carrier for pesticides and fertilizers. Minor uses include filtering, clarifying, and decolorizing, as an active and inactive ingredient in beauty products; and as a filler in paint, plaster, adhesives, and pharmaceuticals. It also has a number of uses in the film industry (fake smoke in simulated explosions) and on stage.
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone (generally called lutites). When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds. Mud is closely related to slurry and sediment. It is used in mud brick construction which is a sustainable building material.
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica (silicon dioxide, or SiO2), usually in the form of quartz. Quartz sand is used in cement mixtures used in construction and concrete pours.
The second most common form of sand is calcium carbonate sand, for example aragonite, which has mostly been created, over the past half billion years, by various forms of life like coral and shellfish. It is, for example, the primary form of sand apparent in areas where reefs have dominated the ecosystem for millions of years, like the Carribean.
Soil is composed of particles of broken rock that have been altered by chemical and mechanical processes that include weathering, erosion and precipitation. Soil is altered from its parent rock due to interactions between the lithosphere, hydrosphere, atmosphere, and the biosphere. It is a mixture of mineral and organic materials that are in solid, gaseous and aqueous states. Soil is commonly referred to as earth or dirt; technically, the term dirt should be restricted to displaced soil.
Soil forms a structure that is filled with pore spaces, and can be thought of as a mixture of solids, water and air (gas). Accordingly, soils are often treated as a three state system. Most soils have a density between 1 and 2 g/cm³. Little of the soil of planet Earth is older than the Tertiary geological period and most of it no older than the Pleistocene. Darkened topsoil and reddish subsoil layers are typical in some regions.
On a volume basis, a good quality soil is one that is 45% minerals (sand, silt, clay), 25% water, 25% air, and 5% organic material, both live and dead. The mineral and organic components are considered a constant with the percentages of water and air being the only variable parameters where the increase in one is balanced by the reduction in the other. A fertile soil will contain all the major nutrients for basic plant nutrition (nitrogen, phosphorus, and potassium), as well as other nutrients needed in smaller quantities (e.g., calcium, magnesium, sulfur, iron, zinc, copper, boron, molybdenum, nickel).
Figure 1. Compositions of various mixtures of clay, sand and silt
An interesting related topic is Geophagia also known as geophagy is the intentional practice of eating earth or soil-like substances such as clay, chalk, or termite mounds. Calabash chalk is a geophagic material popularly consumed in West African countries for pleasure, and by pregnant women as a cure for nausea.
Compiled by Rami E. Kremesti M.Sc., CSci, CWEM, CEnv
References:
1. Assallay A.M., Rogers C.D.F., Smalley, I.J., Jefferson,I. 1998. Earth Science Reviews 45, 61-88
2. Glossary of terms in soil science. Ottawa: Agriculture Canada. 1976. p. 35. ISBN 0662015339.
3. McCarthy, David F. (2006). Essentials of soil mechanics and foundations: basic geotechnics (7th ed.). Upper Saddle River, New Jersey: Prentice Hall. ISBN 978-0131145603.