What it is

Magnetite is an iron oxide with the formula Fe₃O₄, crystallizing in the inverse spinel structure. In simple terms, iron atoms occupy two sublattices whose magnetic moments don’t cancel perfectly—hence a strong net magnetization (ferrimagnetism).

Name & lore

The name derives from the ancient “magnetis lithos” (stone from Magnesia, Greece). Naturally magnetized specimens are called lodestone and were humankind’s earliest compasses—rocks that literally point the way.

Magmatic & cumulate

Magnetite crystallizes early from mafic–intermediate magmas. In layered intrusions, it can concentrate into magnetite–ilmenite bands (titanomagnetite), sometimes forming economic layers.

Metamorphic & skarn

During contact metamorphism of iron‑rich carbonates or shales, magnetite forms in skarns with garnet, pyroxene, epidote, and amphibole—often dense, ore‑grade bodies.

Sedimentary iron formations

In banded iron formations (BIFs), magnetite interlayers with hematite and chert, creating the famous red‑gray stripes that feed much of the world’s iron industry.

Hydrothermal & weathering

Hydrothermal fluids can precipitate magnetite directly; at the surface, magnetite may partially oxidize to maghemite (γ‑Fe₂O₃) and then to hematite.

Biogenic & cosmic cameos

Magnetotactic bacteria grow chains of nanometer‑scale magnetite (“magnetosomes”) to navigate along Earth’s magnetic field. Magnetite also occurs in some meteorites, especially carbonaceous chondrites.

Oceanic recorders

Grains of titanomagnetite in basalts cool and “lock in” the direction of Earth’s field—together, they write the zebra‑striped pattern of magnetic reversals on the seafloor.

Typical look

• Color: iron‑black to steel‑gray.
• Luster: metallic to submetallic; dull where weathered.
• Habit: sharp octahedra, dodecahedra; granular to massive; magnetite “sand” in placers.
• Streak: jet black (very diagnostic).

Crystal details

Faces may show triangular striations or etch pits. Ilmenite exsolution can appear as trellis lamellae (titanomagnetite) in polished sections—catnip for ore microscopists.

Display tip: A small neodymium magnet hidden under the shelf makes loose magnetite sand “stand up” in spiky patterns—instant conversation starter.

Hand‑lens clues

• Metallic black, sometimes with octahedral faces.
• Streak plate gives black line instantly.
• Heft feels high for size (SG ~5.2).

Magnet test

Even small fragments leap to a magnet. Some specimens (lodestone) pick up the magnet back—they carry permanent magnetism.

Polished section

Under reflected light, magnetite is bright and isotropic; tiny exsolution lamellae of ilmenite may create delicate trellis patterns (titanomagnetite).

Hematite

Can be steel‑gray to black, but streak is red. Weak magnetism at most. Specular hematite sparkles; magnetite is more uniformly metallic.

Ilmenite

Iron‑titanium oxide; weakly magnetic to non‑magnetic. Often shows brownish tint and lower density. Streak black but luster less bright.

Chromite

Dark, high‑SG spinel; weak magnetism and brown streak. Common in ultramafics—context helps.

Maghemite & martite

Maghemite (oxidized magnetite) remains black but may be less magnetic; martite is hematite pseudomorph after magnetite—octahedral shape, red streak.

Magnetic slag

Industrial by‑product can be magnetic and glassy with vesicles. Look for bubbly texture and flow swirls (unnatural for mineral crystals).

Quick checklist

• Strong magnet attraction.
• Black streak (decisive test).
• Octahedral crystals or massive granular ore.

Global iron sources

Major iron production comes from banded iron formations (Australia’s Pilbara & Hamersley, Brazil’s Carajás, South Africa, North America’s Lake Superior region) where magnetite and hematite alternate with chert.

Other notable settings

• Magnetite–apatite (IOA) deposits (e.g., Kiruna district, Sweden).
• Skarn magnetite near limestone–granite contacts.
• Layered intrusions with titanomagnetite bands.
• Placer sands (“black sands”) along beaches and rivers.

Iron & industry

Primary ore of iron. Finely ground magnetite also serves as dense media in coal preparation and as a black pigment (Fe₃O₄).

Electronics & materials

Fe₃O₄ nanoparticles sit at the heart of ferrofluids and many ferrites used in cores and RF applications (often with other metal cations).

Earth’s memory

Magnetite grains “tape” the direction and strength of the geomagnetic field in cooling lavas and sediments—key to paleomagnetism and plate‑tectonic reconstructions.

Everyday handling

• Metallic surfaces show fingerprints—wipe with a soft, dry cloth.
• Store separately to avoid scuffing softer neighbors (it’s dense and a bit abrasive).
• Keep strong magnets away from magnetic‑stripe cards and compasses (unless you’re doing demos on purpose!).

Cleaning

• Dust with a soft brush; a slightly damp cloth is fine—dry promptly.
• Avoid acids/bleach; weathered surfaces can redden (oxidize) with harsh treatment.

Easy experiments

• Black‑sand dance: Place a magnet beneath a thin dish of magnetite sand; watch spikes form and move with the magnet.
• Lodestone test: See if your specimen can lift a paperclip by itself—if yes, you’ve got a naturally magnetized piece.
• Streak vs. look‑alike: Compare black magnetite streak with hematite’s red—instant ID confidence.