Ironstone
|
Black-band ironstone, 2.1 billion years old |
Ironstone is a fine-grained, heavy and compact
sedimentary rock. Its main components are the carbonate or oxide of
iron,
clay and/or
sand. It can be thought as a concretionary form of
siderite. Ironstone also contains clay, and sometimes
calcite and
quartz.
Freshly cleaved ironstone is usually gray. The brown external appearance is due to
oxidation of its surface. Ironstone, being a sedimentary rock is not always homogeneous, and can be found in a red and black banded form called
tiger iron, sometimes used for
jewelry purposes.
Sometimes ironstone hosts
concretions or
opal gems.
Ironstone mostly occurs in the form of nodules. It is easy to find ironstone overlying
coal measures.
Ironstone is a bountiful and widespread source of
iron (Fe), although it only contains <50% iron, far less than the other main source of iron,
hematite. Most of British iron originates from ironstone, but it is rarely used for this purpose elsewhere.
*
Iron ore*
Banded iron formation
