Magnetite (Fe3O4): Magnetic Mineral
- Sylvia Rose
- Apr 4
- 6 min read
Magnetite is a magnetic mineral found in diverse geological settings. Its strong magnetism is an predominant quality. Magnetite makes up 10% of the Earth's crust. It's used in ancient compasses and modern tech.
In humans, magnetite exists in the brain, heart, liver and spleen. Ion level imbalance can cause memory dysfunction. Abnormal concentrations are linked to neurodegenerative disorders like Alzheimer's disease.
About Magnetite
Magnetite is a black or brownish-black, opaque mineral belonging to the spinel group. It forms dark, metallic, and strongly magnetic crystals. They range in color from black to a rich brown, often with a shiny, metallic luster.
Its name is derives from Magnesia, a region in ancient Thessaly (Greece) where it's first discovered. Magnetite is a common ore of iron, found in a variety of igneous, metamorphic, and sedimentary rocks.
It's found in igneous and metamorphic rocks, as well as sedimentary deposits. With hematite, magnetite is one of the primary ore minerals for iron.
Formation of Magnetite
Igneous: It crystallizes directly from cooling magma and is often concentrated in layered intrusions.
Metamorphic: It forms during regional or contact metamorphism of iron-rich rocks.
Sedimentary: Magnetite precipitates from iron-rich solutions in sedimentary settings; its results from iron-rich waters undergoing weathering and erosion.
It forms as detrital grains derived from weathering of pre-existing magnetite-bearing rocks. Magnetite can accumulate in riverbeds or lake bottoms as detrital grains.
Biogenic: Some bacteria, known as magnetotactic bacteria, produce magnetite crystals intracellularly. These crystals can then be incorporated into sediments.
Magnetotactic bacteria (MTB) are widespread, motile, diverse prokaryotes. They biomineralize a unique organelle, the magnetosome. Magnetosomes consist of a nano-sized crystal of a magnetic iron mineral, enveloped by a lipid bilayer membrane.
Magnetosomes enable the bacteria to sense and align with Earth's magnetic field. They use it to navigate and find preferred microaerophilic or low-oxygen environments.
Magnetite also forms by hydrothermal process, whereby hot mineral-rich water circulates through rocks. Mineral deposits are found in hydrothermal veins of the Andes mountains, where magnetite crystals develop over time.
Scientific Properties of Magnetite
Composition: 72% iron and 28% oxygen by weight
Color: Black or brownish-black
Streak: Black
Luster: Metallic to dull
Hardness: 5.5 – 6.5 on the Mohs scale
Specific Gravity: 5.15 – 5.18
Melting Point: 1590 °C (2894 °F)
Density: With a density of 5.2 g/cm³, magnetite is heavy compared to many other minerals, largely due to high iron content.
Crystal System: Isometric (cubic), forming octahedral crystals and granular masses. It crystal system directly affects its magnetic behavior.
Fracture: Uneven
Tenacity: Brittle
Magnetism: Strongly magnetic; sometimes exhibits polarity. In polarity an entity has two separate and opposing poles. These can either attract or repel one another. Magnetite is the strongest naturally occurring magnetic mineral.
Inverse Spinel Structure
Magnetite has an inverse spinel structure, which contributes to its unique magnetic properties. Inverse spinel structures have a different cation distribution.
Cations are positively charged ions. A cations are larger and B cations are small. In magnetite, all the A cations and half the B cations occupy octahedral sites. The other half of the B cations occupies tetrahedral sites.
An octahedral site is a type of interstitial site (or "hole") where an atom can reside, surrounded by six other atoms to form an octahedron.
A interstitial site is a position within a crystal structure normally unoccupied by atoms or ions. It can be occupied by other atoms or ions. This is also a factor in electrical conductivity.
In a tetrahedral site, the interstitial atom is at the center of a tetrahedron formed by four lattice atoms. Three of the atoms, in contact with each other, are in the same plane.
The fourth atom is positioned symmetrically above them. This site has a specific geometry providing space for an interstitial atom.
Magnetism
Magnetite's magnetism comes from its unique crystal structure and the behavior of its iron atoms. Iron atoms have a magnetic moment due to the spin of their electrons.
A magnetic moment, or magnetic dipole moment, expresses the strength and direction of a magnet or any object or system generating a magnetic field. This indicates its inclination to align with an external magnetic field.
In most minerals, magnetic moments are random, canceling each other out. In magnetite, each iron atom has unpaired electrons.
Unpaired electrons show magnetism, a phenomenon called paramagnetism. The unpaired electrons' magnetic moments align with an external magnetic field, causing a weak attraction.
Iron atoms occupy two different sites within the crystal lattice. This causes unequal distribution of magnetic moments and creates a magnetic field. Arrangement of iron ions enables strong interaction of the magnetic fields.
Lodestones are natural magnets, exhibiting polarity. They attract iron filings and deflect a compass needle
Ferromagnetic minerals maintain magnetism, even when an external magnetic field is removed. This property is used in modern technologies, including magnetic recording devices.
High-strength magnets are used in everyday electronic devices, as in hard disc drives. They help create magnetic storage systems for digital data.
Iron & Rock Magnetism
Magnetism comes from the alignment of atomic magnetic moments within a material. Iron is a crucial element for magnetism because it has unpaired electrons. This contribute to a strong magnetic moment.
Other rocks with iron-bearing minerals, such as pyrrhotite (iron sulfide), or ilmenite (iron titanium oxide) are weakly magnetic. Degree of magnetism in a rock depends on concentration and alignment of magnetic minerals.
Iron content is an important factor. Specific mineralogy and alignment of magnetic domains within the rock are equally influential.
Magnetic minerals are classified into three categories. These are diamagnetic, paramagnetic and ferromagnetic.
Diamagnetic minerals are repelled by an external magnetic force, and may even levitate. It's the opposite of paramagnetism, in which objects are attracted to external magnetic fields, like magnets and fridges.
Ferromagnetic materials like magnetite have overlapping magnetic domains aligning in the same direction when exposed to a magnetic field. Rocks of low iron content have weaker properties, seeming non-magnetic.
Iron content determines a rock's magnetic behavior. Even a small increase in iron concentration can significantly enhance ferromagnetic traits in minerals.
Uses of Magnetite: From Ancient Compasses to Modern Technology
Ancient Navigation: Lodestones, naturally magnetic magnetite rocks, were used as compasses by ancient civilizations for navigation, particularly by the Chinese.
Iron Ore: As a rich iron ore, magnetite is a primary source of iron for steel production. It remains a primary iron ore for steel manufacturing, accounting for over 30% of global steel output.
Heavy Media Separation: Its density allows it to be used in heavy media separation to separate valuable minerals from waste rock.
Magnetic Recording: Magnetite particles are used in magnetic recording tapes and hard drives to store data.
Medical Applications: Magnetite nanoparticles are used in targeted drug delivery, magnetic resonance imaging (MRI) contrast agents and hyperthermia cancer treatment.
Environmental Remediation: Magnetite nanoparticles can remove pollutants from water and soil. Magnetite is used as a filtration medium to purify water.
Facts about Magnetite
Maghemite: Oxidation of magnetite causes formation of maghemite (γ-Fe₂O₃), another magnetic iron oxide mineral.
Paleomagnetism: Magnetite crystals in rocks record the direction and intensity of the Earth's magnetic field at the time of their formation, providing valuable data for paleomagnetic studies.
The Magnetic Stones at Castle Frankenstein in Germany add to the site's mystic allure. The Castle is also rumored to have a fountain of youth.
Environmental Contribution: Magnetite is important in soil formation and nutrient cycling, contributing to health and fertility of ecosystems.
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