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The black crystals in this picture are a magnetic mineral called magnetite!
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Magnetite

Magnetite is a natural magnet! Part of a mineral group called the oxides, magnetite can usually be identified by its strong magnetism and dark color.

Magnetite is often not very abundant, but it can be found in many different types of igneous, metamorphic, and sedimentary rocks. It can even be found in some meteorites. Most igneous rocks that form deep underground contain a small amount of magnetite crystals. Magnetite can also be found in metamorphic rocks that formed from iron-rich sedimentary rocks.

Black sand grains at beaches and elsewhere are often made of magnetite. If you find a beach that has black sand, try running a magnet over the surface and see whether the little black sand grains stick to it. Over time, those sand grains may cement together forming a sedimentary rock.

Why is magnetite so magnetic?

Any magnetic field is caused by a flow of negatively charged particles called electrons. In magnetite, electrons flow between two different types of iron that have different changes, to even out the changes. This flow of electrons within magnetite generates the magnetic field.

  • Shape: Cubic (typically forming octahedron shapes)
  • Luster: Metallic to sub-metallic
  • Color: Gray, brown to black
  • Streak: Black
  • Hardness: 5.5 to 6.5 on Mohs Hardness Scale
  • Cleavage: None
  • Fracture: Conchoidal

Last modified June 12, 2003 by Lisa Gardiner.

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The Fall 2010 issue of The Earth Scientist, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!

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