Hematite (Fe2O3) is an iron oxide mineral. It is widespread in nature, especially in sedimentary environments. Hematite is one of the two principal iron ores. The other is magnetite, which is also an iron oxide mineral. The term ‘hematite’ itself might not be familiar to everyone, but its rusty red color definitely is. Sandstones are often reddish. This is because sand grains are coated with fine-grained hematitic powder. If soil is reddish, then it is also because of hematite. Even red granite owes its color to this mineral. Iron rust itself is similarly colored, but it is mostly composed of hydrated iron oxides.
Hematitic iron ore with goethite (yellow). Fine-grained hematite is dark red, but it has a silvery sheen when crystalline. These colors are both present in the photo above. Svinsås, Norway. Width of sample 13 cm.
Hematite as a widespread natural pigment has been used by man since the earliest times. Red color in cave paintings comes from fine hematitic powder which is known as red ochre. The use of it as a coloring agent continues to this day. Sweden is an especially noteworthy country in this regard because lots of wooden houses there are traditionally brownish red (Falu red). The leftovers of copper mining from Falun and other mines have been used for making red paint there since the 16th century.
But far more important is the use of hematite as an ore of iron. The majority of it is mined from banded iron formations (BIF). These are old (formed generally more than 2 billion years ago) layered metasedimentary iron-silica rocks where the iron-bearing layers are composed of either hematite or magnetite. BIF is very important rock not only as an iron ore, but also because its genesis is closely related to the oxygenation of the Earth’s atmosphere. It is believed that the world ocean used to contain much more dissolved iron which was precipitated as an insoluble iron oxide (and formed an iron-rich layer of BIF) when iron combined with oxygen produced by photosynthetic cyanobacteria. Oxygen started to accumulate in the atmosphere when the iron level in the ocean was reduced to much lower level. So the formation of hematitic iron ore was a necessary precursor that made multicellular oxygen-breathing life possible.
Hematite is a very important mineral as a principal iron ore. Majority of it is mined from banded iron formations like the sample above from Kryvyi Rih, Ukraine. Dark layers are rich in hematite. Width of sample 7 cm.
Rich, layered sedimentary hematitic iron ore. Ores like that may form when the original silica has been leached. Width of sample 8 cm.
Ilmenite is structurally similar oxide mineral, but it contains titanium in addition to iron. The host rock is anorthosite. Blåfjell, Norway. Width of sample 13 cm.
Hematite is structurally similar to ilmenite (FeTiO3) which is a major titanium ore. There is a solid solution between the two, but only at high temperature (above 1000°C). But even at room temperature its crystals usually contain some titanium. Iron may also be partly replaced by aluminum or manganese.
Hematite is interestingly versatile in appearance. The rocks and minerals that contain it tend to obtain reddish color. And the streak on an unglazed porcelain is also always cherry red, but crystals of hematite are actually silvery gray with a strong metallic luster. Such crystals are known as specular hematite (also specularite and iron-glance) because of better light reflectance. The habit of crystals is versatile as well. It may form botryoidal kidney ores which have a radial fibrous structure (pencil ore) in the interior of the crystal. Hematite may be platy (micaceous hematite). Such crystals easily give off small reddish flakes which soil the fingers when handled. Hematite may be an alteration product of magnetite and it may retain its crystal shape. If this is the case the material is known as martite (hematite pseudomorphs after magnetite). Hematite itself may be altered to hydrous iron oxides although it is fairly stable in the weathering environment as is normal for a mineral which itself is usually a product of weathering.
Crystals typically have a radial structure. Width of sample 6 cm.
Radial aggregates are known as pencil ore. Width of sample 5 cm.
Hematite may be platy. This sample gives off small micaceous flakes and it also shows radial structure in the middle. Width of sample from Morocco 8 cm.
Crystal aggregates have a botryoidal and very reflective surface. Such crystals are known as iron-glance or specularite. Width of sample 6 cm.
Hematite may be weakly magnetic, but usually it does not react to a hand magnet at all. The best diagnostic feature is color. Although the crystals are steel gray the edges if thin enough still demonstrate reddish brown color and crystals that are large enough to show metallic luster are generally suitable for the streak test as well. Goethite is compositionally similar but has a duller brown streak. Cinnabar may be superficially similar because of red color, but it is rarer mineral with much restricted occurrence and it is heavier.
Although hematite is chiefly a mineral of sedimentary environments, it can also crystallize directly from late-stage magmatic fluids. Primary hematite usually occurs in felsic igneous rocks like syenite, granite, trachyte, and rhyolite. The majority of it occurs in (meta)sedimentary rocks like sandstone, banded iron formations, and quartzite.
Hematite is a mineral that gives a reddish color to the soil. Here is an outcrop of laterite in Northern Ireland near the Giant’s Causeway.
Jasper is a reddish impure silica-rich rock. Red color is due to microscopic hematite impurities. Svinsås, Norway. Width of sample 13 cm.
Jasper with micaceous hematite and magnetite. Svinsås, Norway. Width of sample 10 cm.
Bauxite is a principal ore of aluminum. Hematite is not the main mineral phase here, but it is enough to give it a strongly reddish color. Width of sample 8 cm.
Igneous rocks like syenite are often red because alkali feldspar very often contains finely dispersed hematite inclusions. Kiruna, Sweden. Width of sample 10 cm.
It is also responsible for the reddish brown coloration in obsidian. Width of sample 11 cm.
Sandstone contains various amounts of hematite which may give it a visibly layered appearance. Width of the sample from Scotland 7 cm.
Sand grains are usually composed of quartz which is generally not a colorful mineral. They look red because they are partly covered with a very fine-grained hematitic powder. This powder is composed of iron that was once in the crystal structure of minerals like pyroxenes and amphiboles, which are not stable in the weathering environment. Quartz and hematite as resistant minerals remain and form a very common mineral association. Sand sample is from Australia. Width of view 20 mm.
Quartzite as a metamorphosed sandstone may be also reddish because of it. Width of sample 9 cm.
Reddish volcanic glass from California. Width of view 20 mm.
Weakly cemented Devonian sandstone outcrop in Estonia has a reddish hue because of a small amount of hematite it contains.
Red variety of chalcedony is known as carnelian. The sample from Kazakhstan is 14 cm in width.