Magnetite

Magnetite crystals. The width of the view is 25 mm.

Magnetite is a common mineral in rocks and sediments. It is strongly magnetic and therefore easily recognizable. Magnetite grains in sediments are usually very small and metallic black. Magnetite occurs sometimes in octahedral crystals, just like diamond, but most sand grains are rounded.

Magnetite is moderately susceptible to weathering but is nevertheless common constituent of sand and in some places beach sand may be so concentrated in magnetite that it could be used used as an iron ore. In New Zealand a sand deposit called Ironsand is used to make steel. However, magnetite is usually extracted from metasedimentary rocks called BIFs (banded iron formation), not from beach sand.

Magnetite is an iron oxide (Fe₃O₄). Such chemical formula may seem to be impossible. Oxygen has an oxidation state of -2, and iron usually have oxidation states of +2 or +3 (ferrous and ferric iron). To form a crystal, these oxidation states must balance or cancel each other out but 4x-2=-8 which is not balancing 6 (2×3) or 9 (3×3). Is there an error in the formula?

Not really. To overcome this problem it is useful to treat magnetite as a mixture of two iron oxides with oxidation states of +3 and +2 respectively (Fe₂O₃ and FeO) which are combined in a certain way and form magnetite crystals. It is still important to understant that magnetite is not a mixture in the strict sense. It is a crystalline solid in which different iron atoms are chemically combined with oxygen atoms.

Black pyramidal magnetite crystals in a metamorphic rock (phyllite) from Fichtelgebirge, Germany. The width of the sample is 11 cm. The specimen belongs to the Museum of Geology of the University of Tartu.

That's how magnetite sand aligns itself in the presence of a strong external magnetic field. There is a neodymium magnet placed beneath the sample. Magnetite crystals from Talofofo Beach, Guam, USA. The width of the view is 10 mm.

The presence of divalent and trivalent iron ions in the crystal lattice of magnetite is the reason why magnetite is so strongly magnetic. It is actually the most magnetic mineral. Divalent (+2) and trivalent (+3) iron ions have unequal magnetic moments that are therefore not balancing each other.

Some of the trivalent iron is often replaced by titanium. It is still magnetite but sometimes it is called titanomagnetite to stress the titanium content of it. Magnetite may also contain Mg, Mn, Zn, and Ni but usually in small quantity.

Magnetite has very important role in the discovery of plate tectonics but this story of magnetic stripes and seafloor spreading is told so many times that I prefer not to make an attempt to repeat it here one more time.

Magnetite is widely distributed in igneous rocks. In mafic rocks magnetite tends to be titaniferous (Ti-magnetite). Cumulate magnetite layers may sometimes form in magma chambers. Magnetite is also a common metasomatic mineral. It means that magnetite is introduced to the rocks by fluids from igneous intrusions. Rocks that contain such magnetitic iron ores are known as skarns. Magnetite may also form in regionally metamorphosed rocks where it forms at the expense of iron hydroxides (goethite, limonite) and oxides (hematite).

Banded iron formation (metasedimentary rock) from Kryvyi Rih, Ukraine. Black stripes are composed of magnetite. The width of the sample is approximately 10 cm.

This is sand from the North Island of New Zealand. It is used as an iron ore. The black grains are titanomagnetite (total titanium content of the sample is 4 percent). Iron makes up 20 percent of the sample (XRF data). Yellow grains are silicate minerals. The width of the view is 10 mm.