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Magnetite-bearing rock refers to any sedimentary or metamorphic rock that contains a significant proportion of the mineral magnetite (Fe3O4). In sedimentary contexts, this often manifests as Banded Iron Formations (BIFs), which are distinctive layered rocks composed of alternating bands of iron oxides (including magnetite) and chert. In metamorphic contexts, magnetite can be a major constituent in rocks like magnetite schist, magnetite gneiss, or skarns, where it forms through the recrystallization of iron-rich sediments or metasomatic processes. The presence of magnetite imparts a characteristic dark color and magnetic property to the rock.
How to Identify
- Color
- Typically dark gray to black, often with a metallic sheen from magnetite grains. Sedimentary varieties (BIFs) show alternating dark (magnetite/hematite) and light (chert) bands.
- Luster
- Dull to submetallic in the rock matrix, with individual magnetite grains exhibiting a metallic to submetallic luster.
- Texture
- Variable. Sedimentary forms (BIFs) are finely laminated to banded. Metamorphic forms can be fine-grained to coarse-grained, foliated (schistose/gneissic) or massive, depending on the metamorphic grade and protolith.
- Crystal Form
- Magnetite crystals within the rock matrix are typically anhedral to subhedral, often disseminated or concentrated in layers. Euhedral octahedra or dodecahedra can occur in some metamorphic settings.
- Cleavage
- Magnetite itself has no true cleavage but exhibits parting on {111}. The rock matrix may show cleavage or foliation depending on its metamorphic history.
- Geological Environment
- Sedimentary: Ancient shallow marine environments, often associated with anoxic conditions and microbial activity (BIFs). Metamorphic: Regional or contact metamorphic terrains, derived from iron-rich sediments, mafic/ultramafic rocks, or skarn deposits.
Key Facts
- Hardness: 5.5-6.5 (Mohs scale for magnetite)
- Specific Gravity: 4.9-5.2 (for magnetite); the rock's specific gravity will vary based on magnetite content and matrix minerals.
- Crystal System: Isometric (for magnetite)
- Color: Dark gray to black
- Luster: Metallic to submetallic (magnetite); dull to submetallic (rock)
- Transparency: Opaque
- Fracture: Conchoidal to uneven (for magnetite); variable for the rock matrix
- Cleavage: None (for magnetite, but exhibits parting on {111})
- Composition: Iron oxide (Fe3O4) within a matrix of other minerals (e.g., quartz, chert, silicates, carbonates)
Quick Check
- Color: Dark gray to black
- Luster: Dull to submetallic (rock), metallic to submetallic (magnetite grains)
- Streak: Black
Physical Characteristics
- Crystal Habit: Octahedral, dodecahedral, granular, massive (for magnetite)
- Cleavage Type: None (parting on {111})
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Metallic to submetallic
Formation
Magnetite-bearing sedimentary rocks (e.g., Banded Iron Formations) form through chemical precipitation of iron oxides and silica from ancient oceans, often linked to microbial activity and changes in atmospheric oxygen. Magnetite-bearing metamorphic rocks form from the metamorphism of iron-rich sedimentary rocks or other iron-bearing protoliths under conditions of elevated temperature and pressure, leading to recrystallization and growth of magnetite.
Usage
Primary ore for iron production. Magnetite is a significant source of iron. Magnetite-bearing rocks are also used as aggregate, in heavy media separation, and as a pigment. Due to its magnetic properties, it has niche applications in magnetic recording and as a dense medium in coal washing.
Age Distribution
Precambrian to Cenozoic, with significant occurrences in the Archean and Proterozoic Eons for Banded Iron Formations.
Where to Find
Lake Superior Region, USA and Canada
World-class deposits of Banded Iron Formations (BIFs), particularly the Mesabi Range in Minnesota, USA, and the Labrador Trough in Canada, are major sources of magnetite-bearing taconite.
Hamersley Basin, Western Australia
Contains vast deposits of Archean and Proterozoic BIFs, forming some of the largest iron ore reserves globally.
Kiruna, Sweden
Known for its large, high-grade magnetite-apatite iron ore deposits, formed through magmatic or hydrothermal processes.
Brazil (Minas Gerais)
Significant deposits of high-grade iron ore, including magnetite-rich varieties, are found in the 'Iron Quadrangle'.
China (Anshan, Bayan Obo)
Numerous iron ore deposits, including magnetite-rich BIFs and other types, are found across China.
Finding Tips
Magnetic Test
The most definitive field test is to use a strong magnet. Magnetite is strongly ferromagnetic and will attract a magnet. This distinguishes it from hematite, which is only weakly magnetic or non-magnetic.
Color and Streak
Look for dark gray to black rocks. The streak of magnetite is black, which can help differentiate it from hematite (reddish-brown streak).
Geological Context
Search in areas known for ancient sedimentary basins (for BIFs) or metamorphic terrains. Look for layered or banded structures in sedimentary rocks, or dark, dense, and magnetic rocks in metamorphic settings.
Density
Magnetite-bearing rocks are typically denser than average rocks due to the high specific gravity of magnetite. This can be noticeable when handling samples.
Similar Rocks
Hematite-bearing rock
Hematite (Fe2O3) in a sedimentary or metamorphic rock matrix
Also known as: Iron-rich sedimentary rock, Hematite schist/gneiss
Taconite
Chert and fine-grained iron oxides (magnetite, hematite)
Also known as: Low-grade iron ore
Serpentinite
Serpentine group minerals (e.g., antigorite, chrysotile, lizardite) with accessory magnetite
Also known as: Serpentine rock
Scientific Classification
- Mineral Class
- Oxides
- Group
- Spinel Group
- Crystal System
- Isometric
- Chemical Formula
- Fe3O4
- Composition
- Iron (72.36%), Oxygen (27.64%)
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