Hematite (“keel”) is a heavy, red to purplish red, dull to glistening
mineral which leaves a red mark or streak when rubbed on a hard white
rock (like chert) or on unglazed porcelain. This red color of hematite
coating or stain is responsible for our red clays, red soils, red iron
rust, reddish creek-water, and almost every bit of natural, red mineral
matter in Missouri. Hematite is iron oxide, Fe₂O₃, and has a close
associate, limonite, which is yellow to brown in color, and has the
chemical composition Fe₂O₃·nH₂O. The two are mentioned together here
because they are commonly associated in nature, where they can be
recognized in mixture by the yellowish red or reddish brown colors on
rocks or soils. Individual discussion is given limonite under its
heading, but its relationship to hematite is repeated here for obvious
reasons.
[Illustration: Hematite: glistening, fine-grained, and dark red.]
[Illustration: Hematite makes a deep red mark or streak on a hard
white rock or unglazed porcelain.]
Hematite varies in hardness enough that some specimens can be scratched
easily with iron, whereas others are almost as hard as that metal
itself. Where clay occurs mixed with hematite, as in paint ore, it may
be quite soft, but “blue kidney ore” is usually hard.
Hematite is the ore (iron ore) mineral at Iron Mountain and Pilot Knob
mining districts and in the various sink-hole mines or pits in south
central Missouri. Scattered boulders of hematite occur in non-commercial
quantities within a shaly layer (lower part of Pennsylvanian age rocks)
which crops out extensively in central Missouri, and the finding of
these boulders has at times, unfortunately, stimulated short-lived hopes
of locating a valuable deposit of iron ore. Flaming red soil or
mountains of red solid rock (as are present in western United States)
may be colored by less than five per cent iron oxide and are in no sense
iron ore because the iron is not concentrated. Iron is the fourth most
abundant element in the earth’s crust, but workable iron mines and
deposits are few and far between. To be commercially valuable an iron
ore deposit must contain tens of thousands of tons and be relatively
free from impurities, notably sulphur and phosphorus. Hence, not many
Missouri farms are locations of iron ore deposits.
The origin of Missouri hematite is about as diverse as its occurrences.
Hot iron-rich solutions coming from an igneous source below are believed
to have introduced the hematite in the Iron Mountain-Pilot Knob area,
but the sink-hole hematite resulted from the oxidation of iron sulphide.
Weathering of older iron-containing rocks and minerals gave rise to the
coloring hematite seen on our sub-soil and surface rock.
Hematite is used as a polishing agent, as a pigment in paint, and, of
course, as an ore of metallic iron. In the smelting of iron from
hematite the ore is mixed in a huge, chimney-shaped blast furnace with
coke (from coal) and limestone. Air is blown into the furnace as into a
blacksmith’s forge; and the coke and gasses, burning at an incandescent
heat, take the oxygen from the Fe₂O₃, leaving metallic iron which melts
and is run out of the furnace at periodic intervals. Thus the smelting
process is the opposite of the rusting process. The impurities and
cinder run out as molten slag.