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Ironstone

Pictures of Sedimentary Rock Types

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Ironstone is a name for any sedimentary rock that is cemented with iron minerals. There are actually three different kinds of ironstone, of which this one is the most typical. (more below)
Rusty-looking rock
Photo courtesy Joey Gabaldon, all rights reserved

The official descriptor for an ironstone is ferruginous ("fer-ROO-jinus"), so you could also call these specimens ferruginous shale—or mudstone; I can't really tell. This ironstone is cemented together with reddish iron oxide minerals, either hematite or goethite or the amorphous combination called limonite. It typically forms discontinuous thin layers or concretions, and both can be seen in this collection. There may also be other cementing minerals present such as carbonates and silica, but the ferruginous part is so strongly colored that it dominates the rock's appearance.

Another type of ironstone, also called clay ironstone, occurs associated with carbonaceous rocks like coal. The ferruginous mineral is siderite (iron carbonate) in that case, and it's more brown or gray than reddish. It contains a lot of clay, and whereas the first kind of ironstone may have a tiny amount of iron oxide cement, clay ironstone has a substantial amount of siderite. It too occurs in discontinuous layers and concretions (which may be septaria).

The third main variety of ironstone is better known as banded iron formation, best known in large assemblages of thin-layered semimetallic hematite and chert. It formed during Archean time, billions of years ago under conditions unlike any found on Earth today. In South Africa, where it's widespread, they may call it banded ironstone, but a lot of geologists just call it "biff" for its initials BIF.

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