Properties, uses, and occurrence of the most important ore of iron.
Article by: Hobart M. King, Ph.D., RPG
What is Hematite?
Hematite is one of the most abundant minerals on Earth's surface and in the shallow crust. It is an iron oxide with a chemical composition of Fe2O3. It is a 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 production comes from 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.
Hematite has a wide variety of other uses, but their economic significance is very small compared to the importance of iron ore. The mineral is used to produce pigments, preparations for heavy media separation, radiation shielding, ballast, and many other products.
Physical Properties of Hematite
|Color||Black to steel-gray to silver; red to reddish brown to black|
|Streak||Red to reddish brown|
|Luster||Metallic, submetallic, earthy|
|Mohs Hardness||5 to 6.5|
|Specific Gravity||5.0 to 5.3|
|Diagnostic Properties||Red streak, specific gravity|
|Uses||The most important ore of iron. Pigment, heavy media separation, radiation shielding, ballast, polishing compounds, a minor gemstone|
Physical 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 reddish streak. Students in introductory geology courses are usually surprised to see a silver-colored mineral produce a reddish streak. They quickly learn that the reddish streak is the most important clue for identifying hematite.
Hematite is not magnetic and should not respond to a common magnet. However, many specimens of hematite contain enough magnetite that they are attracted to a common magnet. This can lead to an incorrect assumption that the specimen is magnetite or the weakly magnetic pyrrhotite. The investigator must check other properties to make a proper identification.
If the investigator checks the streak, a reddish streak will rule out identification as magnetite or pyrrhotite. Instead, if the specimen is magnetic and has a reddish streak, it is most likely a combination of hematite and magnetite.
Composition of Hematite
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 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" (see image).
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 hematite 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 in the water. 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. The new oxygen immediately combined with the iron to form hematite, which sank to the bottom of the seafloor and became the rock units that we know today as the banded iron formations.
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 iron 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.
Many of the sedimentary iron deposits contain both hematite and magnetite as well as other iron minerals. These are often in intimate association, and the ore is mined, crushed, and processed to recover both minerals. Historically, much of the hematite was not recovered and was sent to tailings piles. More efficient processing today allows more hematite to be recovered from the ore. The tailings can also be reprocessed to recover additional iron and reduce tailings volume.
Hematite on Mars?
NASA has discovered that hematite is one of the most abundant minerals in the rocks and soils on the surface of Mars. An abundance of hematite in Martian rocks and surface materials gives the landscape a reddish brown color and is why the planet appears red in the night sky. It is the origin of Mars' "Red Planet" nickname.
Uses of Hematite (Iron Ore)
Hematite is the world’s most important ore of iron. Although magnetite contains a higher percentage of iron and is easier to process, hematite is the leading ore because it is more abundant and present in deposits in many parts of the world.
Hematite is mined in some of the largest mines in the world. These mines require investments of billions of dollars, and some will remove over 100 million tons of ore per year. These open-pit mines can be hundreds to thousands of feet deep and several miles across by the time they have been worked to completion.
China, Australia, Brazil, India, Russia, Ukraine, South Africa, and the United States are the world’s leading producers of iron ore (includes hematite, magnetite, and other ores). Iron ore production in the United States occurs in Michigan and Minnesota.
Uses of Hematite (Pigment)
The name hematite is from the Greek word "haimatitis" which means "blood-red." That name stems from the color of hematite when it has been crushed to a fine powder. Primitive people discovered that hematite could be crushed and mixed with a liquid for use as a paint or cosmetic. Cave paintings, known as "pictographs," dating back to 40,000 years ago were created with hematite pigments.
Hematite continues to be one of the most important pigment minerals. It has been mined at many locations around the world and has been traded extensively as a red pigment. During the Renaissance when many painters began using oils and canvas, hematite was one of the most important pigments. Hematite color was opaque and permanent. It could be mixed with a white pigment to produce a variety of pink colors that were used to paint flesh.
Uses of Hematite (Gem Material)
Hematite is a minor gem material used to produce cabochons, beads, small sculptures, tumbled stones, and other items. The material used to manufacture these products is a silver-colored hematite with a solid, uniform texture. The bright silver color of hematite and its "weighty feel" make it a very popular tumbled stone.
Uses of Hematite (Healing Stone)
Some people believe that carrying pieces of tumble-polished hematite, known as "healing stones," will bring relief from certain medical problems. There is no scientific proof that this use of hematite has any positive effect beyond being a placebo. Using hematite as a "healing stone" or a "healing crystal" can actually be harmful because it diverts people from seeing a doctor who can provide proper care. Then when the person with the problem finally decides to see a doctor, their situation is more severe.
Other Uses of Hematite
Hematite is used for a number of other purposes. It is a very dense and inexpensive material that is effective at stopping x-rays. For that reason it is used for radiation shielding around medical and scientific equipment. The low cost and high density of hematite and other iron ores also makes them useful as ballast for ships.
Hematite can also be ground to a fine powder that when mixed with water will make a liquid with a very high specific gravity. These liquids are used in the "float-sink" processing of coal and other mineral material. The crushed coal, which has a very low specific gravity, is placed on the heavy liquid and the light clean coal floats, while high-specific-gravity impurities such as pyrite sink.
Finally, hematite is the material used to make polishing compounds known as "red rouge" and "jeweler's rouge." Red rouge is a hematite powder used to polish brass and other soft metals. It can be added to crushed corn cob media or crushed walnut shell media for tumble-polishing brass shell casings. Jeweler's rouge is a paste used on a soft cloth to polish gold and silver jewelry.
|Mohs Hardness Scale|
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