It is usually formed from the chemical weathering of iron rich minerals, such as biotite, amphibole, pyroxene, and olivine, the hydration/oxidation of iron rich sulfide minerals, or from the hydration of magnetite and hematite. Limonite is commonly deposited in run-off streams resulting from mining operations.

Why is limonite not a mineral?

This research revealed that the material called "limonite" does not meet the definition of a mineral. Instead, limonite is a mineraloid composed mainly of hydrous iron oxides that are often found in intimate associations with iron minerals.

Is limonite a form of rust?

Humid to semihumid climates readily oxidize metallic iron and iron minerals to limonite (ferric hydroxide), common rust. Limonite is a mixture of finely crystalline goethite (the alpha-FeOOH) and the amorphous form of the same composition and color.