How to identify Iron-rich Sedimentary Rock

To correctly identify Iron-rich Sedimentary Rock (Ferruginous Sedimentary Rock), 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 yellowish-brown due to the presence of iron oxides (hematite, goethite) or iron carbonates (siderite).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes sub-metallic in the case of magnetite-rich varieties.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable, ranging from fine-grained (shale, mudstone) to medium-grained (sandstone) or even conglomeratic. Often clastic, but can be chemical or biochemical. Banded varieties (BIFs) show distinct layering.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Iron minerals within the rock are typically microcrystalline or amorphous, rarely exhibiting macroscopic crystal forms. Hematite may occur as specularite (platy crystals) in some veins or metamorphic settings.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Generally absent in the bulk rock, though individual mineral grains (e.g., siderite) may exhibit cleavage. The rock itself may show parting along bedding planes.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine environments (shallow to deep), lacustrine environments, bog iron deposits, and weathered profiles. BIFs are characteristic of ancient Precambrian oceans.

Key Facts

Hardness

Variable, depending on the dominant iron mineral and cementing agent. Hematite: 5-6, Magnetite: 5.5-6.5, Goethite: 5-5.5, Siderite: 3.5-4.5. The overall rock hardness will be influenced by the matrix.

Specific Gravity

High, typically ranging from 2.9 to 4.5, significantly higher than most common sedimentary rocks, due to the density of iron minerals.

Crystal System

Not applicable to the rock as a whole. Constituent iron minerals have their own crystal systems (e.g., Hematite: trigonal, Magnetite: isometric, Goethite: orthorhombic, Siderite: trigonal).

Color

Reddish-brown, dark red, black, yellowish-brown.

Luster

Dull to earthy, sub-metallic.

Transparency

Opaque.

Fracture

Conchoidal to uneven, depending on the mineralogy and texture.

Cleavage

Generally absent in the rock mass; individual mineral grains may exhibit cleavage.

Composition

Predominantly iron oxides (hematite Fe2O3, magnetite Fe3O4, goethite FeO(OH)), iron hydroxides (limonite, a mixture of hydrated iron oxides), iron carbonates (siderite FeCO3), and iron silicates (e.g., chamosite, greenalite), often interbedded or mixed with chert (SiO2), quartz, clay minerals, or carbonates.

Physical characteristics

• Crystal Habit: Not applicable to the rock. Constituent iron minerals can be massive, earthy, oolitic, botryoidal, or crystalline.
• Cleavage Type: Absent in the rock mass. Individual minerals may show cleavage (e.g., siderite has perfect rhombohedral cleavage).
• Fracture Type: Conchoidal, uneven, or splintery.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, sub-metallic.

Explore Iron-rich Sedimentary Rock