Plinthite (from the Greek plinthos, brick) is an iron-rich, humus-poor mixture of clay with quartz and other minerals.
The product of pedogenesis, it commonly occurs as dark red redox concretions that usually form platy, polygonal, or reticulate patterns.
Plinthite changes irreversibly to an ironstone hardpan or to irregular soil aggregates on exposure to repeated wetting and drying, especially if it is also exposed to heat from the sun.
The lower boundary of a zone in which plinthite occurs generally is diffuse or gradual, but it may be abrupt at a lithologic discontinuity.
These concretions are not considered plinthite unless there has been enough segregation of iron to permit their irreversible hardening on exposure to repeated wetting and drying.