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Magnetite is a common iron oxide mineral and one of the most important iron ores. It is characterized by its strong magnetism, which is a key diagnostic property. It typically occurs as black, opaque, metallic to submetallic crystals or granular masses. Its chemical formula is Fe3O4, representing a mixed iron(II,III) oxide. It is a member of the spinel group of minerals.
How to Identify
- Color
- Black to brownish-black.
- Luster
- Metallic to submetallic.
- Texture
- Typically granular, massive, or as distinct octahedral or dodecahedral crystals. Can also be found as fine grains.
- Crystal Form
- Cubic system, commonly forming octahedral crystals (eight-sided, double pyramids) or dodecahedral crystals (twelve-sided). Also occurs as granular masses.
- Cleavage
- None, though it may exhibit parting on {111} planes.
- Geological Environment
- Igneous rocks (e.g., gabbros, basalts, granites), metamorphic rocks (e.g., skarns, banded iron formations, amphibolites), and sedimentary rocks (e.g., placers, ironstones). Also found in hydrothermal veins.
Key Facts
- Hardness: 5.5-6.5 on the Mohs scale
- Specific Gravity: 5.17-5.18
- Crystal System: Isometric (Cubic)
- Color: Black to brownish-black
- Luster: Metallic to submetallic
- Transparency: Opaque
- Fracture: Conchoidal to uneven
- Cleavage: None, but parting on {111} is common
- Composition: Iron oxide (Fe3O4)
Quick Check
- Color: Black to brownish-black
- Luster: Metallic to submetallic
- Streak: Black
Physical Characteristics
- Crystal Habit: Octahedral, dodecahedral, granular, massive
- Cleavage Type: None, but parting on {111} planes due to twinning or exsolution lamellae
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Metallic to submetallic
Formation
Magnetite forms in a wide variety of geological environments. It is a common accessory mineral in igneous and metamorphic rocks, and can also form in sedimentary environments. In igneous rocks, it crystallizes from magma as an early-forming mineral. In metamorphic rocks, it forms during regional or contact metamorphism of iron-rich sediments or other rocks. In sedimentary environments, it can accumulate as detrital grains in placers or form through diagenetic processes. Hydrothermal processes can also deposit magnetite.
Usage
Magnetite is the most important iron ore mineral, used primarily for steel production. Its magnetic properties make it useful in various industrial applications, including magnetic recording media, pigments, and as a heavy aggregate in concrete. Naturally magnetized magnetite (lodestone) was historically used as the first compasses.
Age Distribution
Found in rocks of all ages, from Precambrian to Cenozoic, depending on the geological setting.
Where to Find
Kiruna, Sweden
One of the world's largest and richest magnetite deposits, primarily associated with igneous iron ore.
Krivoy Rog, Ukraine
Significant deposits of magnetite within banded iron formations.
Hamersley Basin, Western Australia
Vast banded iron formations containing significant magnetite, though hematite is often more dominant.
Adirondack Mountains, New York, USA
Historically important magnetite deposits in metamorphic rocks.
Siberia, Russia
Large deposits associated with various geological settings.
Finding Tips
Use a Magnet
The most definitive test for magnetite is its strong magnetism. A simple hand magnet will readily stick to it. This distinguishes it from other black, metallic minerals.
Check the Streak
Magnetite produces a black streak on an unglazed porcelain plate. This helps differentiate it from hematite (red streak) and chromite (brownish-black streak).
Observe Crystal Habit
Look for characteristic octahedral or dodecahedral crystal forms, especially in vugs or areas of good crystal growth.
Consider Geological Context
Magnetite is common in iron-rich igneous and metamorphic rocks, as well as in heavy mineral sands (placers). Knowing the local geology can aid in identification.
Similar Rocks
Hematite
Hematite
Also known as: Specularite, Kidney Ore, Red Ochre
Chromite
Chromite
Also known as: Chrome Ore
Ilmenite
Ilmenite
Also known as: Titanium Iron Oxide
Scientific Classification
- Mineral Class
- Oxides
- Group
- Spinel Group
- Crystal System
- Isometric (Cubic)
- Chemical Formula
- Fe3O4
- Composition
- Iron(II,III) oxide (FeO·Fe2O3 or FeO.Fe2O3)
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