Generally hard, compact, ellipsoidal mineral
masses in a sedimentary or volcanic rock, with a composition
different from the host rock. Most have regular smooth
shapes, but some have irregular or odd-shaped boundaries.
Concretions commonly are made of carbonate minerals in a
sandstone matrix or chert in limestone. Other common concretions
include pyrite, iron oxides, and gypsum. Most concretions
are several centimeters to a few decimeters in
diameter, but some are as large as a few meters across. Concretions
are thought to form from precipitation from aqueous
solutions, and they often precipitate on a nucleus made of an
unusual mineral, fossil, or grain.
See also DIAGENESIS.
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