Magnetite
The most magnetic naturally occurring mineral, a major iron ore, and the reason the compass exists.
Magnetite is iron oxide with both ferrous and ferric iron in the same structure, and it is strongly magnetic — the only common mineral that is. Some specimens, called lodestone, are permanently magnetised and will attract iron filings and align themselves north-south when suspended.
That behaviour is why the compass exists. A lodestone needle was the first magnetic direction finder, and navigation by magnetic bearing rather than by sight of land follows directly from a property of this one mineral.
How to identify it
A magnet settles it immediately: nothing else common responds that strongly. Combined with a black streak, metallic lustre and high density, the identification is close to unambiguous.
Hematite, the other major iron ore, is weakly magnetic at best and leaves a distinctly red-brown streak — the streak test separates the two reliably even when both look black and metallic in hand specimen.
Uses
Magnetite is a principal iron ore, and its magnetism is an industrial advantage: it can be concentrated by magnetic separation, which is cheaper and cleaner than the flotation methods other ores require. Taconite pellets, the feedstock of many blast furnaces, are made from low-grade magnetite ore upgraded this way.
Its density and magnetism also make it useful in its own right — as a dense medium for separating coal from rock, as a component of some concretes for radiation shielding, and, finely divided, in magnetic recording media and in ferrofluids.
How it forms
Magnetite crystallises early from cooling magma and accumulates in layered intrusions, and it forms in metamorphic rocks and hydrothermal systems. Its most economically important occurrence is in banded iron formations — enormous Precambrian sedimentary deposits laid down when oxygen first accumulated in the oceans and precipitated dissolved iron.
Those formations are the source of most of the world's iron ore, and they record an event that happened once and cannot happen again, because the oceans no longer carry dissolved iron in that quantity.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is an ore of
- Iron — element · higher iron content than hematite, and concentrated magnetically rather than by flotation Wikidata
is commonly confused with
is used as
- Pigment — application · as a black iron oxide pigment, and historically in inks
is a component of
- Banded iron formation — rock · 10–50% · the other iron layer, dominant in the unweathered rock and the reason taconite can be separated magnetically
- Skarn — rock · the ore of the iron skarns, formed where an oxidised intrusion met carbonate rock
- Gabbro — rock · in the upper layers of a layered intrusion, where vanadium-bearing magnetitite seams form
- Carbonatite — rock · a common accessory, and at Bayan Obo the reason the deposit was originally worked as an iron mine
- Serpentinite — rock · produced by the reaction itself, from the iron released as olivine breaks down; it is why serpentinite is magnetic
is an alternative to
- Hematite — mineral · as iron ore. Hematite ships direct at high grade where it has been weathered rich; magnetite is leaner and separates magnetically at low cost, which is what makes taconite worth mining at all — the choice is about the concentrator, not the metal
belongs to the group
- Ferrite — material · the naturally occurring one, and the reason anybody knew magnetism existed — lodestone is a ferrite, and the engineered ones are its spinel structure with other metals substituted in
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain