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THE MINERAL MAGNETITE


Magnetite is an oxide of iron (as is hematite). It is not a component of ordinary rust, although it can form as iron oxidizes in a dry environment. When you see sparks from welding operations or from iron striking a hard surface (as when it is held against a spinning grinder), the sparks are due to the rapid (and exothermic) oxidation of tiny particles of iron into magnetite.

Magnetite is a natural magnet, hence the name, giving it a very nice distinguishing characteristic. Explaining the magnetism is not easy but here is a go at it. Remember, electricity produces magnetic fields just as magnetism produces electric fields. Magnetite is a member of the spinel group which has the standard formula A(B)2O4. The A and B represent usually different metal ions that occupy specific sites in the crystal structure. In the case of magnetite, Fe3O4, the A metal is Fe +2 and the B metal is Fe +3; two different metal ions in two specific sites. This arrangement causes a transfer of electrons between the different irons in a structured path or vector. This electric vector generates the magnetic field.

Magnetite is the primary oxide of iron found in meteorites (and thus presumably in asteroids).

Thermite is a mixture of powdered magnetite and powdered aluminum. When ignited, the aluminum fuel burns using the magnetite as an oxidizer, resulting in much heat, aluminum oxide (corundum), and molten iron.

PHYSICAL CHARACTERISTICS:

  • Color is black.
  • Luster is metallic to dull.
  • Transparency: Crystals are opaque.
  • Crystal System is isometric; 4/m bar 3 2/m
  • Crystal Habits are typically octahedrons but rarely rhombododecahedron and other isometric forms, most commonly found massive or granular. Twinning of octahedrons into spinel law twins is seen occasionally.
  • Cleavage is absent although octahedral parting can be seen on some specimens.
  • Fracture is conchoidal.
  • Hardness is 5.5 - 6.5
  • Specific Gravity is 5.1+ (average for metallic minerals)
  • Streak is black.
  • Associated Minerals are talc and chlorite (schists), pyrite and hematite.
  • Other Characteristics: Magnetism stronger in massive examples than in crystals, striations on crystal faces (not always seen).
  • Notable Occurrences include South Africa, Germany, Russia and many localities in the USA.
  • Best Field Indicators are magnetism, crystal habit and streak.