Magnetite, otherwise known as Ferrous-Ferric Oxide or Magnetic iron ore is the other main ore of iron, in addition to Hematite, and has a chemical formula of Fe3O4. It was named after the Magnesia region of Thessaly, Greece and the word “magnet” actually comes from the mineral rather than vice versa. Another story was that Magnes, a Greek shepherd, first discovered the mineral when he noticed that the nails of his shoe stuck to a rock. It is the most magnetic naturally occurring mineral found on earth and small pieces of the ore can become magnetised naturally and will attract bits of iron which is how magnetism itself was originally discovered. Small grains of Magnetite are found in most igneous and metamorphic rock and is also, less commonly, present in some sedimentary rock, sometimes in banded iron formations. In appearance, it is a black, opaque, submetallic to metallic mineral and has a Mohs hardness between 5.5 and 6.5. It is often found in the form of isometric crystals.
The magnetic nature of the ore has led to lodestones, the naturally magnetised version of the mineral, to be used in early compasses. It has also been useful in helping people understand the conditions under which rocks form and the movement of plate tectonics because the particles that it is made up of orient themselves in the direction of the earth’s magnetic field when magnetite is created in igneous and sedimentary rock so changes in the field over time can be studied by looking at the orientation of magnetic particles over time. As well as being an ore of iron, it can also be used as an abrasive – Emery is a mix of magnetite and corundum, for example. The ore is also used in industry, as a catalyst in the industrial synthesis of Ammonia and as a coating for steam boilers due to its stability at high temperatures. It can also be used as a sorbent to remove Arsenic from drinking water. Other uses include being used as a toner in electrophotography, as a micronutrient in fertilizers, as a pigment in paints, and as an aggregate in high-density concrete.
Magnetite is a commonly found mineral and in addition to the mineral rock form, it is also found as a black sand when it is eroded by rivers and deposited on a beach. Black sand is notably present in New Zealand and California. Magnetite does have a lower iron content than Hematite which means it has to undergo more processing to make it suitable for steel making. The magnetic properties of the ore means that it is particularly suitable for the further processing of iron ore into iron ore pellets or refined into concentrate, however. While it is generally a lower-grade deposit, it is globally accepted as a viable and high-quality feedstock for the production of premium quality, low impurity steel. The processing route for the ore requires crushing, screening, grinding, magnetic separation, filtering and drying. After this processing, however, this ore does end up having a higher iron content than benchmark direct shipping ore which means that iron ore sellers can recoup some of the extra costs associated with the higher processing required comparative to hematite.
Photo credit to R.Weller/Cochise College.