Limonite: Nautral Rust

Limonite a term which refers to an un-identified mixture of hydrated iron oxides and hydroxides--it is pretty much just naturally-occurring rust. Limonite is amorphous meaning that it has no crystal shape, and therefore it can’t really be considered a mineral, but I have never heard it classified as a rock. Perhaps it falls into the mineraloid category. Limonite is the color of rust and is found as massive, earthy lumps. For the most part is has an earthy or dull luster, which means that it does not reflect light. Limonite’s amorphous structure seems to be the only thing that separates it from goethite another mineral composed of iron oxides and hydroxides. Goethite forms fibrous crystals.
Limonite is famous for staining things with its yellow-brown color. In fact when you pick up a piece it will probably leave a dusty brown residue on your hands. Limonite is a coloring-agent in some soils and rocks, and it often colors agates and cherts. Sometimes this coloration occurs only on the surface forming a rust-colored crust, but it occasionally penetrates deeper. If you ever look for rocks in Lake Michigan or Lake Huron you will likely come across a great many chert pebbles that have been colored by limonite.
Limonite usually forms when other iron-bearing minerals (such as hematite) weather upon exposure. Since limonite is composed of hydrated iron oxide it will revert to hematite when it dries out. It also forms what is known as “bog ore”. Bog ore is limonite that forms as chunks below the water in marshlands and lakes.
Limonite was once used to make a kind of paint called “ochre”. It may serve as a minor ore of iron but I am not sure about that. It can be found theoretically wherever iron is present and seeing as iron is the fourth most plentiful element in the earth’s crust there are a great many potential sources.