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Home » Minerals » Magnetite

Magnetite and Lodestone


An ore of iron, a mineral used in heavy media separation and a recorder of Earth magnetism


What is Magnetite?



Magnetite is one of the most common oxide minerals and also one of the most common iron minerals. It is an important ore of iron and is found in igneous, metamorphic and sedimentary rocks. It can also be abundant in sediments.


Identification of Magnetite



Magnetite is easy to identify. It is a black, opaque, submetallic to metallic mineral with a Mohs hardness between 5.5 and 6.5. It is often found in the form of isometric crystals. However, its magnetic properties are distinctive. It is one of just a few minerals that are attracted to a magnet. It is the most magnetic mineral found in nature. Sometimes it is automagnetized and attracts metal objects.


Magnetite as "Lodestone"



Lodestone is a form of magnetite that acts as a natural magnet. Normal magnetite is attracted to a magnet but lodestone acts as a magnet, attracting iron particles (see photo).


Physical Properties of Magnetite

Chemical Classification oxide
Color Black
Streak Black
Luster Metallic to submetallic
Diaphaneity opaque
Cleavage none
Mohs Hardness 5.5 to 6.5
Specific Gravity 5.2
Diagnostic Properties Strongly magnetic, color, streak, octahedral crystal habit
Chemical Composition Fe3O4
Crystal System Isometric
Uses


Use of Magnetite As An Ore of Iron



Most of the magnetite mined is used as an ore of iron. Iron liberated from the ore is usually used to make iron, steel and other alloys.


Use of Magnetite as a Heavy Media



Powdered magnetite is often mixed with a liquid for used as a heavy media for specific gravity separations. Much of the high sulfur coal that is mined is floated across a slurry of magnetite. Clean coal particles float and those contaminated with pyrite (a sulfide mineral with a high specific gravity) sink into the high-density slurry.


Use of Magnetite as an Abrasive



The abrasive known as "emery" is a natural mixture of magnetite and corundum. Some synthetic emery is produced by mixing magnetite with aluminum oxides. Producing it synthetically allows control over the particle size and the relative abundance of aluminum oxide and magnetite. Some finely ground magnetite is also used as an abrasive in water jet cutting. In the past few decades synthetic abrasives have filled many of applications where magnetite was previously used.


Other Uses of Magnetite



Other uses include: as a toner in electrophotography, as a micronutrient in fertilizers, as a pigment in paints, as an aggregate in high-density concrete.


Magnetite and Earth's Magnetic Field



Tiny crystals of magnetite are present in many rocks. In the crystallization of an igneous rock, tiny crystals of magnetite form in the melt, and because they are magnetic, they orient themselves with the direction and polarity of Earth's magnetic field. This preserves in the rock the orientation of Earth's magnetic field at the time of crystallization.

Today geologists can study the magnetic properties of rocks of various age and reconstruct the history of change in Earth's magnetic field. This information is available for multiple locations on multiple continents. It can also be used to learn about the movement of continents over time.

A similar orientation of tiny magnetite grains occurs in the settling of sediment particles, locking clues to Earth's magnetic history into some sedimentary rocks.


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Typical magnetite. Specimen is approximately four inches (10 centimeters) across.




lodestone magnetite
Lodestone: note now the specimen attracts small particles of iron. Specimen is approximately four inches (10 centimeters) across.


magnetite sand
Magnetite sand. Some beach and river sands contain high concentrations of magnetite. Magnetite sand obtained by dredging a stream and separating out the heavy minerals. Pile is approximately four inches (10 centimeters) across.


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