MAGNETITE [ Oxides and Hydroxides : Spinel ]

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.

Physical Characteristics

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)

Associated Minerals: talc and chlorite (schists), pyrite and hematite

Major Occurrences: include South Africa, Germany, Russia and many locallities in the USA

Best Indicators: magnetism, crystal habit and streak

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