What does peloid mean?
A peloid is a small, irregularly shaped particle or aggregate of calcium carbonate found in carbonate rocks and sediments. The term is especially associated with sedimentology, petrography, and palaeontology, where researchers examine the shapes, composition, and internal structure of microscopic grains to understand how sediments formed. Peloids are commonly composed of micrite, a very fine-grained calcium carbonate material, and may contain traces of organic material or other sediment trapped during their growth. The word is useful because shape and internal texture provide evidence about the processes that produced a sedimentary rock. Its distinction from a pellet is important: although the two may both be small carbonate aggregates, a pellet is usually rounded, whereas a peloid is characteristically irregular. Outside these technical fields, the word is uncommon and should not be used as a general synonym for pellet or particle.
nounAn irregularly shaped grain or small aggregate of calcium carbonate, commonly composed of micrite and found in carbonate sediments.
- A small, irregularly shaped aggregate of carbonate sediment, especially one composed of micrite.
- A fine carbonate particle or aggregate whose shape results from the aggregation or accretion of smaller particles.
"The thin section revealed several small peloids embedded in the limestone matrix."
"Microscopic peloids make up much of the matrix of the limestone bed."
"The petrographer identified the grains as peloids rather than rounded pellets or broken shell fragments."
The standard plural adds -s; peloids may be counted individually or described collectively.
"The thin section contains hundreds of tiny peloids."
A peloid is a tiny piece of carbonate sediment whose name literally means “mud-like” — a reminder that a single grain can preserve a whole history of the ancient sea.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does peloid come from?
The word was formed in the early 20th century from Greek pelē, meaning “mud” or “clay,” together with the combining form -oid, meaning “resembling” or “like.” The same Greek root underlies pelite, a sedimentary rock composed largely of clay, and related geological terminology. Its development follows the period when sedimentologists increasingly distinguished carbonate grains by their shape and origin.
How peloid is actually used
This is a specialist geological term rather than everyday vocabulary. In sedimentology, “pellet” and “peloid” are related terms but are not always interchangeable: pellets are typically rounded, while peloids are characteristically irregular.
Easily confused with peloid
A pellet is generally a rounded, usually faecal sedimentary aggregate, whereas a peloid is distinguished chiefly by its irregular shape.