Ironstone is a
sedimentary rock, either deposited directly as a ferruginous
sediment or created by chemical replacement, that contains a substantial proportion of an
iron ore
Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...
compound from which
iron (Fe) can be smelted commercially. Not to be confused with native or
telluric iron, which is very rare and found in metallic form, the term ''ironstone'' is customarily restricted to hard, coarsely banded, non-banded, and non-
cherty sedimentary rocks of
post-Precambrian age. The
Precambrian
The Precambrian (or Pre-Cambrian, sometimes abbreviated pêž’, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
deposits, which have a different origin, are generally known as
banded iron formations. The iron minerals comprising ironstones can consist either of oxides, i.e.
limonite
Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·H2O, although this is not entirely accurate as the ratio of oxide to hydroxid ...
,
hematite
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
, and
magnetite; carbonates, i.e.
siderite; silicates, i.e.
chamosite; or some combination of these minerals.
[U.S. Bureau of Mines Staff (1996) ''Dictionary of Mining, Mineral, & Related Terms.'' Report SP-96-1, U.S. Department of Interior, U.S. Bureau of Mines, Washington, D.C.][Neuendorf, K. K. E., J. P. Mehl Jr., and J. A. Jackson, J. A., eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ]
Description
Freshly cleaved ironstone is usually grey. 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.
Occurrence
Ironstone occurs in a variety of forms. The various forms of ironstone include siderite nodules; deeply weathered
saprolite
Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops its color comes from ferric compounds. Deeply weathered profiles are widespread o ...
, i.e. (
laterite
Laterite is both a soil and a rock type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by ...
); and
ooidal ironstone.
Uses
Ironstone as a source of iron
Ironstone, although widespread, is a limited source of iron. Historically, most British iron originated from ironstone, but it is now rarely used for this purpose because it is far too limited in quantity to be an economic source of iron ore.
Ceramics
Ironstone's oxide impurities render it useless as a component in
ceramics: the "
ironstone china" of Staffordshire and American manufacture, a fine white high-fired vitreous