[Illustration: Dark brown limonite.]
[Illustration: Stalactitic limonite from southeastern Missouri.]
Limonite is a heavy, yellow to brown, or brownish black mineral which
always leaves a yellow to brown mark or streak when rubbed across a hard
white rock or unglazed porcelain. It usually has a dull luster on a
broken surface, and may vary from thumb-nail hardness to almost that of
steel. The distinguishing test is its yellow to brown streak.
In composition limonite is iron oxide which contains more or less water
chemically combined, Fe₂O₃·nH₂O. That is, it may be dried bone-dry at
the temperature of boiling water, but upon heating to redness the
additional, chemically held water will be driven off.
Limonite is ordinarily formed from the weathering of other
iron-containing minerals (pyrite, for example) and is therefore a
wide-spread mineral in surface rock, in films on pools of water, and in
soil, all of which it colors yellow to brown. In fact, almost all of the
yellow to brown inorganic mineral color and stain seen in nature is that
of limonite.
Commercial deposits of limonite occur in southeastern Missouri where
large boulders, discontinuous and irregular lenses or beds, pipes,
nodules, and gravel to clay-sized particles of the mineral are
associated with the cherty, gravelly residual clay. Usually the ore is
crushed, hand-sorted, and washed preparatory to concentration for
shipment to a furnace or for use in cement manufacture. As in the case
of hematite, unless one has a deposit amounting to thousands of tons of
ore it has little commercial value, and unless the mineral is relatively
pure it can not be used.