Ironstone is an ancient medium- to fine-grained sedimentary
rock containing iron-rich minerals such as hematite and siderite; magnetite and pyrite may also be present. Ironstones
were formed two to three billion years ago in the Precambrian era. In banded ironstone, the basic rock is chert interspersed with layers of ferruginous minerals. Oolitic limestone,
formed in the bottom of sediments at the bottom of ancient seas, consists
of small rounded grains (ooliths) in which
ferruginous minerals have replaced lime-rich ones. In both types, the iron-containing
minerals result in a deep red color.
AND PLANETARY SCIENCE