A chemical sedimentary rock, typically thin-bedded and/or finely laminated, containing at least 15% iron of sedimentary origin, and commonly but not necessarily containing layers of chert. Various primary facies (usually not weathered) of iron formation are distinguished on the basis of whether the iron occurs predominantly as oxide, silicate, carbonate, or sulfide. Most iron formation is of Precambrian age. In mining usage, the term refers to a low-grade sedimentary iron ore with the iron mineral(s) segregated in 1643 bands or sheets irregularly mingled with chert or fine-grained quartz. Compare: ironstone; jaspilite. Essentially synonymous terms are itabirite; banded hematite quartzite; taconite; quartz-banded ore; banded iron formation; calico rock; jasper bar.