LIMONITE is an iron oxide containing water and has a complex chemical
composition. The limonite found in Illinois may be yellow, orange, red,
brown, or black, but its streak is always yellowish brown. The mineral
may have a glassy or an earthy luster. It may be too hard to be
scratched by a knife. It is of medium weight.
Limonite is common and occurs as concretions and cavity fillings in
sedimentary rocks, and as coatings on them, especially sandstone. It
also occurs as iron rust, as scum on stagnant water, and it accumulates
around rootlets in soils. Small amounts color limestone, dolomite, clay,
shale, sandstone, and gravel. Some sands are firmly cemented by brown or
black limonite and look much like iron ore. Clays containing a high
percentage of limonite are called ocher.
In some states limonite is mined as an iron ore, and in Illinois it was
so used in Hardin County in the middle 1800’s, but deposits are not
large enough for profitable use now.