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 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 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, hematite, and magnetite; carbonates, i.e. siderite; silicates, i.e. chamosite; or some combination of these minerals.[1][2]
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[broken anchor], sometimes used for jewelry purposes.
Ironstone occurs in a variety of forms. The various forms of ironstone include siderite nodules; deeply weathered saprolite, i.e. (laterite); and ooidal ironstone.
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.
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 semi-porcelain, commonly used for heavy-duty dinner services in the 19th century,[3] includes no ironstone in its production. Its "iron" quality is in its resistance to chipping.
^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. ISBN0-922152-76-4
^G. Bernard Hughes, English and Scottish Earthenware, Abbey Library
