How to identify Iron-rich Sedimentary Rock

To correctly identify Iron-rich Sedimentary Rock (Sedimentary rock with iron oxides (e.g., hematite, goethite)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically reddish-brown, dark red, black, or gray. The color is highly dependent on the dominant iron oxide mineral (e.g., red for hematite, yellowish-brown for goethite, black for magnetite).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes submetallic in areas rich in hematite or magnetite.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Highly variable. Can be finely laminated (Banded Iron Formations), massive, oolitic (spherical grains of iron minerals), pisolitic, or clastic (containing detrital grains). May show evidence of bedding or stratification.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual iron oxide minerals may be microcrystalline to cryptocrystalline. Macroscopic crystal forms are rare in the rock matrix itself, but ooids or pisoids can be spherical to ellipsoidal.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No distinct cleavage for the rock as a whole. Individual mineral components (e.g., hematite, magnetite) have their own cleavage/parting characteristics, but these are not typically observed in the bulk rock.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine (shallow to deep water), lacustrine, or bog environments. BIFs are characteristic of ancient anoxic to euxinic oceans. Phanerozoic ironstones often form in shallow, oxygenated marine settings on continental shelves.

Key Facts

Hardness

Variable, typically 5-6.5 for hematite/magnetite, 5-5.5 for goethite (Mohs scale) within the rock matrix. The overall rock hardness depends on the cementing material and other constituents.

Specific Gravity

High, typically 3.0-5.0, depending on the iron mineral content. Pure hematite is ~5.26, pure magnetite is ~5.18, pure goethite is ~4.27.

Crystal System

Not applicable for the rock as a whole. Constituent minerals have their own crystal systems (e.g., hematite is trigonal, magnetite is isometric, goethite is orthorhombic).

Color

Reddish-brown, dark red, black, or gray.

Luster

Dull to earthy, submetallic.

Transparency

Opaque.

Fracture

Conchoidal to uneven, depending on the mineralogy and texture.

Cleavage

None for the bulk rock. Individual minerals may exhibit cleavage or parting.

Composition

Primarily iron oxides (hematite, magnetite) and hydroxides (goethite), often with silica (chert/quartz), carbonates (siderite), or silicates (chamosite). Iron content can range from 20% to over 60% Fe.

Physical characteristics

• Crystal Habit: Not applicable for the rock as a whole. Constituent minerals can be massive, granular, oolitic, botryoidal, or microcrystalline.
• Cleavage Type: None for the bulk rock. Hematite has no cleavage but distinct parting; magnetite has no cleavage; goethite has perfect {010} cleavage.
• Fracture Type: Variable, typically uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, submetallic.

Explore Iron-rich Sedimentary Rock