Fe3O4, iron Oxide Major ore of iron and as mineral specimens
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 electic 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.
Colour : black
Luster : metallic to dull
Transparency : Crystals are opaque
Crystal System : isometric; 4/m bar 3 2/m
Crystal Habits : typically octahedrons but rarely rhombododecahedron and other isometric forms, most commonly found massive or granular. Twinning of octahedrons into spinel law twins is seen occassionally
Cleavage : absent although octahedral parting can be seen on some specimens
Fracture : conchoidal
Hardness : 5.5 – 6.5
Specific Gravity : 5.1+ (average for metallic minerals)
Streak : black
Other : Magnetism stronger in massive examples than in crystals, striations on crystal faces (not always seen)
Major Occurrences : include South Africa, Germany, Russia and many locallities in the USA
Best Indicators : magnetism, crystal habit and streak