Iron-rich sedimentary rock. The source of the iron is primary and/or diagenetic (see diagenesis), developed by a number of possible processes. These include: (a) replacement of carbonate particles by hematite, siderite, or chamosite; (b) diagenetic development of nodules, and more continuous horizons, of siderite within claystone sequences; (c) primary deposition of ooids from Fe/Al-rich gels, and subsequent conversion to chamosite during shallow burial. See plinthite. See also minette; clinton ironstone.