Iron Ore

Sedimentary Rock (often secondary enrichment)

Iron Ore (likely Limonite/Goethite with Quartz)

Also known as: Bog Iron, Brown Iron Ore, Ochre (when earthy)

Got a photo? Identify rocks instantly with the Rockby app

Description

Iron ore, specifically referring to Limonite/Goethite with Quartz, is a common type of iron ore characterized by its hydrated iron oxide composition. Limonite is not a true mineral but a field term for a mixture of hydrated iron oxides, predominantly Goethite (α-FeO(OH)) and sometimes Lepidocrocite (γ-FeO(OH)). These minerals are typically amorphous or cryptocrystalline. The presence of Quartz (SiO2) indicates a siliceous component, which can range from fine-grained chert to larger detrital quartz grains. This combination often results in a relatively lower iron content compared to hematite or magnetite ores, but it is still a significant source of iron globally. It commonly forms in surficial environments through weathering processes.

How to Identify

Color: Typically yellowish-brown, reddish-brown, dark brown, or black. The color can be highly variable depending on the degree of hydration and impurities. Quartz will appear colorless, white, or grey.
Luster: Earthy, dull, submetallic, or silky (for fibrous Goethite). Quartz will have a vitreous (glassy) luster.
Texture: Often massive, botryoidal, stalactitic, or concretionary. Can be earthy, porous, or compact. The quartz component can be granular, crystalline, or cherty.
Crystal Form: Limonite/Goethite rarely forms distinct crystals; typically massive, reniform, botryoidal, stalactitic, or oolitic aggregates. Quartz forms hexagonal prisms with pyramidal terminations when euhedral, but is often anhedral or granular within the ore.
Cleavage: Limonite/Goethite lacks distinct cleavage. Quartz exhibits no cleavage but has a conchoidal fracture.
Geological Environment: Found in oxidized zones of iron-bearing mineral deposits, bog iron deposits, lateritic soils, and as cementing material in sandstones and conglomerates. Also common in gossans (weathered caps of sulfide deposits).

Key Facts

Hardness: Limonite/Goethite: 4-5.5 (Mohs); Quartz: 7 (Mohs)
Specific Gravity: Limonite/Goethite: 2.7-4.3 (variable due to porosity and impurities); Quartz: 2.65
Crystal System: Limonite (amorphous/cryptocrystalline); Goethite: Orthorhombic; Quartz: Trigonal
Color: Yellowish-brown, reddish-brown, dark brown, black (Limonite/Goethite); colorless, white, grey (Quartz)
Luster: Earthy, dull, submetallic, silky (Limonite/Goethite); vitreous (Quartz)
Transparency: Opaque (Limonite/Goethite); Transparent to translucent (Quartz)
Fracture: Uneven to conchoidal (Limonite/Goethite); Conchoidal (Quartz)
Cleavage: None (Limonite/Goethite); None (Quartz)
Composition: Hydrated iron oxides (primarily FeO(OH) for Goethite) mixed with Silicon Dioxide (SiO2)

Quick Check

Color: Yellowish-brown to dark brown/black (Limonite/Goethite); colorless/white/grey (Quartz)
Luster: Earthy, dull, submetallic (Limonite/Goethite); vitreous (Quartz)
Streak: Yellowish-brown to brownish-yellow

Physical Characteristics

Crystal Habit: Limonite/Goethite: Massive, botryoidal, stalactitic, reniform, oolitic, earthy. Quartz: Granular, massive, prismatic crystals (less common in ore).
Cleavage Type: None
Fracture Type: Uneven to conchoidal (Limonite/Goethite); Conchoidal (Quartz)
Tenacity: Brittle
Luster Type: Earthy, dull, submetallic, silky (Limonite/Goethite); Vitreous (Quartz)

Formation

Limonite and Goethite are secondary iron minerals, typically forming from the weathering and oxidation of primary iron-bearing minerals (like pyrite, siderite, or other iron silicates) in the presence of water and oxygen. They can precipitate from iron-rich solutions in bogs, swamps, and shallow marine environments, or form as residual deposits in lateritic soils. Quartz is a ubiquitous mineral and can be present as detrital grains, chert, or secondary silica cement within the iron ore.

Usage

Primarily used as a source of iron for steel production. Historically, it was used as a pigment (ochre) and for medicinal purposes. Lower-grade ores may be used as aggregate or in cement production.

Age Distribution

Can form throughout geological time, but significant deposits are often associated with specific paleoenvironments, such as Precambrian Banded Iron Formations (BIFs) or Cenozoic bog iron deposits.

Where to Find

Mesabi Range, Minnesota, USA

Historically a major source of iron ore, including taconite (a low-grade iron formation containing magnetite and hematite with quartz), but also secondary enrichment zones with hydrated iron oxides.

Krivoy Rog, Ukraine

Significant deposits of iron ore, including both high-grade hematite and lower-grade hydrated iron oxides.

Carajás Mine, Pará, Brazil

Known for high-grade hematite, but weathering processes can lead to the formation of hydrated iron oxides in surficial zones.

Pilbara Region, Western Australia

World-class iron ore deposits, predominantly hematite, but also includes goethite-rich ores formed through supergene enrichment.

Bog Iron Deposits (worldwide)

Found in temperate and boreal regions, forming in swamps, bogs, and shallow lakes where iron-rich groundwater precipitates hydrated iron oxides.

Finding Tips

Look for Rusty Stains

Limonite/Goethite often stains surrounding rocks and soil a characteristic yellowish-brown to reddish-brown color due to its iron content. This can be a good indicator of its presence.

Check for Earthy or Dull Luster

Unlike metallic iron ores like hematite or magnetite, limonite/goethite typically has an earthy, dull, or submetallic luster. The quartz component will be glassy.

Test the Streak

Limonite/Goethite produces a yellowish-brown to brownish-yellow streak, which is a key diagnostic feature to distinguish it from hematite (red streak) or magnetite (black streak).

Consider the Geological Environment

Search in areas with evidence of weathering of iron-rich rocks, bogs, swamps, or areas with gossans (oxidized caps of sulfide deposits).

Assess Hardness

Limonite/Goethite is relatively soft (4-5.5 on Mohs scale), while the associated quartz will be harder (7). This difference can be felt when scratching.

Similar Rocks

Hematite

Fe2O3

Also known as: Red Iron Ore, Specularite

Magnetite

Fe3O4

Also known as: Magnetic Iron Ore

Siderite

FeCO3

Also known as: Iron Carbonate

Pyrite

FeS2

Also known as: Fool's Gold

Scientific Classification

Mineral Class: Oxides and Hydroxides (Goethite); Silicates (Quartz)
Group: Goethite Group (for Goethite); Quartz Group (for Quartz)
Crystal System: Orthorhombic (Goethite); Trigonal (Quartz)
Chemical Formula: FeO(OH) (Goethite) + SiO2 (Quartz) + variable H2O and impurities
Composition: Hydrated iron oxide (Goethite) and Silicon Dioxide (Quartz). Limonite is a mixture, not a distinct mineral, and its exact composition varies.

Explore Iron Ore

Identification Localities & Collecting Look-alikes

Identify rocks anywhere

Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.

Download on theApp Store Get it onGoogle Play Continue onWeb

Open in app