Chemical: Fe2+Fe3+2O4 (repeating unit)
Crystal System: Isometric
Crystal Habit: Octahedral
Color: Black, gray with brownish tint in reflected sunlight
Mohs scale: 5.5 – 6.5
Specific Gravity: 5.17- 5.18
Found in igneous, metamorphic, and sedimentary rocks. It is the most commonly mined ore of iron. It is also the mineral with the highest content of iron at 72.4%. Magnetite is ferrimagnetic and is the most strongly magnetic mineral found in nature. It is attracted to a magnet and can be magnetized to become a permanent magnet itself. A naturally magnetized piece is called lodestone and was used by ancient people as crude compasses.
Since magnetite typically carries the dominant magnetic signature in rocks, it has been a critical tool in paleomagnetism, an important tool in understanding plate tectonics. With the paleomagnetism, we can actually see the changes in the Earth’s magnetic field over the eons.
Magnetite is found as a black sand all over the world. It is carried to the beaches via rivers from erosion. Huge deposits have been found in banded iron formations. These sedimentary rocks have been used to infer changes in the oxygen content of the atmosphere of the Earth.
Most of the iron ore mined today is a banded sedimentary rock known as taconite that contains a mixture of magnetite, hematite, and chert. Once considered a waste material, taconite became an important ore after higher grade deposits were depleted. Today’s commercial taconites contain 25 to 30% iron by weight.
At the mine site, the taconite ore is ground to a fine powder, and strong magnets are used to separate magnetically susceptible particles containing magnetite and hematite from the chert. The concentrate is then mixed with small amounts of limestone and clay, then rolled into small round pellets. These pellets are easy to handle and transport by ship, rail, or truck. They can be directly loaded into a blast furnace at a mill and be used to produce iron or steel.
Along with being used as an iron ore, it is also used as a toner in electrophotography and as a source of micronutrients in fertilizer.
As can be seen in this image, iron dust particles cling to the magnetite through its natural magnetism.
Octahedral crystals are a common habit of magnetite. They are often seen in igneous and metamorphic rocks and sometimes seen in sediments near the magnetite source area. The magnetite crystals in this photo are about eight to twelve millimeters in maximum dimension.
Banded sand with the magnetite and other heavy minerals in quartz sand. Chennai, India.
Some beach and river sands contain high concentrations of magnetite. Magnetite-rich “black sands” are commonly encountered by people panning for gold. Although magnetite sands and other heavy mineral accumulations are common, they are infrequently developed as mineral deposits because their size or grade is inadequate. The pile in the photo is approximately four inches (10 centimeters) across.
Processed Taconite pellets as used in the steelmaking industry, with a US Quarter shown to scale.