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Home » Minerals » Hematite

Hematite


Mineral Properties and Uses



What is Hematite?



Hematite is an iron oxide mineral with a chemical composition of Fe2O3. It is a very common rock-forming mineral found in sedimentary, metamorphic and igneous rocks at locations throughout the world.

Hematite is the most important ore of iron. Although it was once mined at thousands of locations around the world, today almost all of the mining is done at a few dozen large deposits where significant equipment investments allow companies to efficiently mine and process the ore. Most ore is now produced in China, Australia, Brazil, India, Russia, Ukraine, South Africa, Canada, Venezuela and the United States.


Properties of Hematite



Hematite has an extremely variable appearance. Its luster can range from earthy to submetallic to metallic. Its color ranges include: red to brown and black to gray to silver. It occurs in many forms that include: micaceous, massive, crystalline, botryoidal, fibrous, oolitic and others.

Even though hematite has a highly variable appearance it always produces a red streak. Students in introductory geology courses are usually surprised to see a silver-colored mineral produce a red streak. They quickly learn that the red streak is the most important clue for identifying hematite.

Physical Properties of Hematite

Chemical Classification oxide
Color black to steel-gray to silver; red to reddish brown to black
Streak red to reddish brown
Luster metallic, submetallic, earthy
Diaphaneity opaque
Cleavage none
Mohs Hardness 5 to 6 1/2
Specific Gravity 4.9 to 5.3
Diagnostic Properties red streak, specific gravity
Chemical Composition Fe2O3
Crystal System hexagonal
Uses the most important ore of iron, pigment, cosmetic, polish, gemstone

Pure hematite has a composition of about 70% iron and 30% oxygen by weight. Like most natural materials it is rarely found with that pure composition. This is particularly true of the sedimentary deposits where hematite forms by inorganic or biological precipitation in a body of water. Minor clastic sedimentation in can add clay minerals to the iron oxide. Episodic sedimentation can cause the deposit to have alternating bands of iron oxide and shale. Silica in the form of jasper, chert or chalcedony can be added by chemical, clastic or biological processes in small amounts or in significant episodes. These layered deposits of hematite and shale or hematite and silica have become known as the “banded iron formations”.


Geologic Occurrence



Hematite is found as a primary mineral and as an alteration product in igneous, metamorphic and sedimentary rocks. It can crystallize during the differentiation of a magma or precipitate from hydrothermal fluids moving through a rock mass. It can also form during contact metamorphism when hot magmas react with adjacent rocks.

The most important deposits formed in sedimentary environments. About 2.4 billion years ago Earth’s oceans were rich in dissolved iron but very little free oxygen was present. Then a group of cyanobacteria became capable of photosynthesis. The bacteria used sunlight as an energy source to convert carbon dioxide and water into carbohydrates, oxygen and water. This reaction released the first free oxygen into the ocean environment. That oxygen immediately combined with the iron to form hematite, which sank to the bottom of the seafloor.

Soon, photosynthesis was occurring in many parts of Earth’s oceans and extensive hematite deposits were accumulating on the seafloor. This deposition continued for hundreds of millions of years - from about 2.4 to 1.8 million years ago. This allowed the formation of deposits hundreds to several thousand feet thick that are laterally persistent over hundreds to thousands of square miles. They comprise some of the largest rock formations in Earth’s rock record.




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Banded Iron Formation: Close-up of a banded iron formation. In this specimen bands of hematite (silver) alternate with bands of jasper (red). This photo spans an area of rock about one foot wide. Photo taken by André Karwath, GNU Free Documentation License.


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