Limonite is another ore of iron, this time with a chemical formula of FeO(OH)·nH2O, although it can be rather variable in compositions so could have other formulas, and is not considered a true mineral because of the variability of structure and is a mixture of other minerals. A lot of what is considered Limonite is actually Geothite, an iron oxide which is the most common component of iron rust. It is named after the Greek word for meadow as it is sometimes found in bogs and marshes. As such is also otherwise known as bog ore. The ore is quite dense and hardness is within the range of 4 to 5.5, so somewhat lower than Magnetite and the specific gravity is between 2.9 and 4.3, which is slightly higher than average. It ranges in colour from bright yellow to greyish brown but generally looks brownish on a white background.
The mineral is sometimes formed from the hydration of both Hematite and Magnetite; from the chemical weathering of other minerals rich in iron or from the hydration of iron rich sulphide minerals. It is always formed due to the alteration or solution of previously existing iron minerals and therefore, it is often found in the run-off streams from iron ore mines. Limonite is very common worldwide with deposits notable in China, Italy, Spain and the US.
Limonite can be used in the production of iron and steel but this is much less common than for both Hematite and Magnetite. It has been known to contain Nickel, however, and some producers have used the ore to make stainless steel. It has been used extensively as a pigment in the past, forming yellow to brown hues and can still be used as a colouration in paints of other dyes. It has also been used for prospecting as it can sometimes signify the presence of gold ore.