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.