Magnetite
Magnetite is Fe3O4, strongly magnetic, hardness 5.5 to 6.5, density 5.175, black streak, 72.36% iron. Telling it from the other black octahedra and hematite.
Magnetite is iron oxide, Fe3O4 (IMA: Fe2+Fe3+2O4), a spinel-group mineral: cubic, typically octahedral, hardness 5.5 to 6.5, measured density 5.175, black with a black streak, and recorded by the Handbook as strongly magnetic. It is 72.36% iron by mass, and USGS names it with hematite as the primary forms of iron ore.
In short
- Two irons in one formula. The September 2026 IMA-CNMNC Master List writes Fe2+Fe3+2O4, as does the rendered Handbook of Mineralogy sheet: one ferrous to two ferric iron. Worked on standard atomic weights, Fe3O4 is 31.03% FeO and 68.97% Fe2O3, the sheet’s ideal column, and 72.36% iron. The Lovers Pit, Mineville, New York analysis gives a ferric-to-ferrous ratio of 2.01 to 1.
- The streak-and-magnet rule. Of the six black iron-bearing oxides in the table, two sheets print both a black streak and strongly magnetic: magnetite and magnesioferrite. Chromite and jacobsite streak brown, ilmenite black to reddish brown, hematite cherry-red, and all four print weak magnetism or none.
- The documented negative: magnesioferrite. Streak and magnet cannot separate it from magnetite. Weight can, on a clean piece: 20.00 g reads 16.14 g in water as magnetite and 15.60 to 15.70 g as magnesioferrite, at least 0.44 g apart. Hematite, at 16.20 g, is only 0.06 g away, which is why streak goes first.
- An ore of iron, per USGS. The USGS National Minerals Information Center says iron ore almost always consists of iron oxides, the primary forms of which are magnetite and hematite. The Handbook records economic deposits by magmatic segregation or contact metamorphism, and extensive deposits in sedimentary banded iron formations.
- Magnetism, measured. USGS Open-File Report 99-529 pulled magnetite on a laboratory separator at below 0.01 to 0.05 amp, best 0.01, across 6 samples, and classes it as ferromagnetic. The site-wide comparison is on which minerals are magnetic.
| Species | IMA formula | Group or system | Hardness | Density (measured) | 20.00 g piece in water | Streak | Magnetism on the sheet | USGS best range (amp) |
|---|---|---|---|---|---|---|---|---|
| Magnetite | Fe2+Fe3+2O4 | Spinel group; cubic | 5.5 to 6.5 | 5.175 | 16.14 g | Black | Strongly magnetic | 0.01 |
| Magnesioferrite | MgFe3+2O4 | Cubic; octahedra to 5 mm | 6 to 6.5 | 4.55 to 4.65 | 15.60 to 15.70 g | Black | Strongly magnetic | Not tabled |
| Jacobsite | Mn2+Fe3+2O4 | Cubic; octahedra to 4 mm, rarely | 5.5 to 6.5 | 4.76 | 15.80 g | Brown | Weakly magnetic | Not tabled |
| Chromite | Fe2+Cr2O4 | Cubic; octahedra to about 1 cm | 5.5 | 4.5 to 4.8 | 15.56 to 15.83 g | Brown | Some samples are weakly magnetic | 0.30 to 0.40 |
| Ilmenite | Fe2+Ti4+O3 | Hexagonal; thick tabular | 5 to 6 | 4.72 | 15.76 g | Black to reddish brown | Weakly magnetic | 0.20 to 0.30 |
| Hematite | Fe2O3 | Hexagonal; rhombohedral | 5 to 6 | 5.26 | 16.20 g | Cherry-red or reddish brown | None recorded | 0.10 to 0.30 |
What is magnetite? Fe3O4, one ferrous to two ferric iron
The formula, read two ways. Fe3O4 is the short form. The IMA list and the Handbook sheet both write Fe2+Fe3+2O4, which says where the charges sit: one Fe2+ and two Fe3+ for four oxygens. The IMA row is grandfathered, 1845, with the country of first description unknown. The sheet’s Name field: an ancient term, possibly an allusion to the locality, Magnesia, Greece.
Worked on standard atomic weights (Fe 55.845, O 15.999): Fe3O4 has a formula mass of 231.53, of which iron is 72.36%. As oxides that is 31.03% FeO and 68.97% Fe2O3, the sheet’s third analysis column exactly. The Lovers Pit, Mineville, Essex Co., New York analysis, FeO 30.78 and Fe2O3 68.85, gives 72.08% iron and a ferric-to-ferrous ratio of 2.01 to 1. The sheet’s Meier’s Find, Western Australia analysis is by electron microprobe with the ratio calculated from stoichiometry, so working the ratio back from it would only return the assumption; we do not.
Habit and breakage. Typically octahedral, less commonly dodecahedral and striated, to 25 cm; very rarely cubic; also skeletal, granular and massive. Twins follow the spinel law on {111}, as contact twins, flattened or lamellar. It has no cleavage: the sheet records a very good parting on {111} and an uneven fracture. The site’s cleavage or fracture page explains the difference.
Is magnetite magnetic? What the two sources record
The Handbook says Strongly magnetic, on the rendered sheet, in the Physical Properties field. Of the six sheets in this page’s table, it and magnesioferrite are the two that use the word strongly. Jacobsite and ilmenite read weakly magnetic, chromite some samples are weakly magnetic, and hematite records nothing.
USGS measured it. Table 1 of Open-File Report 99-529 gives magnetite a separator range below 0.01 to 0.05 amp, best 0.01, over 6 samples, flagged as often locked with other species. The report defines ferromagnetic as strongly attracted to a magnet, like a piece of iron, and names magnetite first among the common ferromagnetic minerals. Hematite, ilmenite and chromite need 0.10 to 0.40 amp at best, the paramagnetic range.
What this page does not say. Neither source describes natural magnets or lodestone, and lodestone is not a row on the IMA list, so we give no account of it. The tests for a magnetic grain in another specimen are on is tetrahedrite magnetic and is galena magnetic, and are not repeated.
Magnetite or hematite, chromite or ilmenite? The streak-and-magnet rule
Step one, streak. On unglazed porcelain: cherry-red or reddish brown is hematite; brown points to chromite or jacobsite; black to reddish brown, ilmenite; black, magnetite or magnesioferrite. The method is on how to use a streak plate.
Step two, the magnet. Of the two black-streak sheets, both print strongly magnetic; of the four others, none does. Counted over the six in the table: 2 strongly, 3 weakly or some samples weakly, 1 none recorded. A weak response therefore does not identify anything by itself.
Step three, weight, for the one pair the first two steps leave. A clean 20.00 g piece reads 16.14 g in water as magnetite and 15.60 to 15.70 g as magnesioferrite: 0.44 g or more apart. The method is on how to measure specific gravity at home. Weight is useless against hematite, 16.20 g, only 0.06 g away, as the west Cumbria hematite page also found; that is why streak comes first. Magnesioferrite’s sheet names volcanic localities such as Vesuvius, Etna and Stromboli, and Magnet Cove, Arkansas, which is also on magnetite’s list.
Where magnetite is found: Sweden, the Urals, Binntal and Itabira
Occurrence, per the sheet: a common accessory mineral in igneous and metamorphic rocks; magmatic segregation or contact metamorphism may produce economic deposits; extensive deposits in sedimentary banded iron formations; a biogenic product; important detrital deposits. Its associates include chromite, ilmenite and apatite in igneous rocks, pyrrhotite, pyrite, chalcopyrite and hematite in hydrothermal and metamorphic ones, and hematite and quartz in sediments.
The Distribution field opens many localities, even for fine crystals, and names Falun, Kiruna and Västanfors, Sweden; Arendal, Norway; Zlatoust and Magnitogorsk, Ural Mountains, Russia; the Zillertal, Austria; Traversella, Italy; Binntal and Rimpfischwang, Valais, Switzerland; the Gardiner complex, Greenland; Bancroft, Ontario; Lake Sanford and the Tilly Foster mine, New York; Magnet Cove, Arkansas; the Iron Springs district, Utah; the Cerro del Mercado, Durango, Mexico; and Itabira, Minas Gerais, Brazil.
No British locality is on the magnetite sheet, and we add none. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.