Ironstone
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Black-band ironstone, 2.1 billion years old.

Close-up of Banded Iron Formation specimen from Upper Michigan. Scale bar is 5.0 mm.

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.

Appearance

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 mostly occurs in the form of nodules, for example in seams in the coal measures.

Uses

Due to its unique properties, ironstone was once popular for use in English tea sets

Ironstone as a source of iron

Ironstone is a bountiful and widespread source of iron (Fe), although it only contains less than 50 percent iron, far less than the other main source of iron, hematite. Historically, most of British iron originated from ironstone, but it is now rarely used for this purpose.

See also

• Iron ore
• Banded iron formation

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