Magnetite Composition, Crystallization & Structure

Composition. Fe₃O₄ or FeO.Fe₂O₃ = Iron sesquioxide 69.0, iron protoxide 31.0 or oxygen 27.6, iron 72.4. The ferrous iron is sometimes replaced by magnesium, rarely nickel; also at times titaniferous.

Crystallization. Isometric. Octahedral habit, sometimes twinned octahedrons. Dodecahedron at times either alone or with octahedron. Other forms rare.

Structure. Usually granular massive, coarse or fine, sometimes as sand; also frequently crystallized.

Physical Properties. Often under pressure develops octahedral parting. H. 0 6.  G. = 5.18. Metallic luster. Color iron-black. Streak black. Strongly magnetic; sometimes a natural magnet, know as lodestone.

Test. Infusible. Slowly soluble in HCI and solution reacts for both ferrous and ferric iron. Distinguished chiefly by its strong magnetism, its black color and streak, and its hardness (6)

Occurrence. A common ore of iron. It is found as an accessory mineral in rocks of all classes and sometimes becomes their chief constituent. Most commonly associated with crystalline metamorphic rocks, also frequently in rocks that are rich in ferromagnesium minerals, such as diabase, gabbros, periditite. In many cases forms large or ore bodies that are thought to be the result of magmatic at times in immense beds and lenses, enclosed in old metamorphic rocks. Found in the black sands of the seashore. Occurs as thin plates and dendritic growths between plates of mica. Often intimately associated with corundum, forming the material known as emery.

In the United States, found in large beds with the Archaean rocks of the Adirondacks in Warren, Essex and Clinton counties of northern New York; in various places in New Jersey; at Cornwall, Pennsylvania. Important foreign localities are in Norway and Sweden, where it is the chief iron ore. Natural magnets or lodestones are found in Siberia; in the Harsh Mountains, Germany; at Magnet Cove, Arkansas.

Name. probably derived from the locality Magnesia, bordering on Macedonia. A fable, told by Pliny, ascribes its name to a shepherd named Magnes, who first discovered the mineral on Mount Ida by noting that the nails of his shoes and the iron ferrule of his staff adhered to the ground.

Use. An important iron ore.