Limonite is an ore of iron, and it causes coloration in soil. The widespread and
common hydrated iron oxide mineral limonite is a minor ore
of iron and a source of ocher and umber pigments. Having no
crystalline form and containing highly variable amounts of
water, limonite forms yellowish earthy coatings or brown to
blackish, stalactitic, grapelike, or fibrous masses and
concretions. Hardness is 4-5.5, luster is silky or earthy,
streak is yellowish brown, and specific gravity is 2.7-4.3.
Much of the material formerly thought to be limonite now is
known to be goethite, which is crystalline and has a
definite chemical composition. Limonite, always formed
from the oxidation (weathering) of other iron minerals, is
common in the gossan capping of sulfide deposits, in
marshes as bog iron ore, and in oolites.