How to identify Iron-stained rock

To correctly identify Iron-stained rock (Ferruginous 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

   Ranges from bright yellow, orange, and reddish-brown to dark brown and black, depending on the specific iron oxide/oxyhydroxide present and its hydration state. Hematite typically imparts red hues, while goethite/limonite imparts yellow to brown hues.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Typically dull to earthy, but can be sub-metallic if the iron oxides are well-crystallized or abundant.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable, as it is a staining phenomenon. The texture of the host rock (e.g., sandstone, granite, shale) will dominate. The iron staining itself may appear as a fine coating, a cement, or disseminated specks.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Iron oxides/oxyhydroxides typically occur as cryptocrystalline to microcrystalline coatings or amorphous masses. Individual crystals are rarely macroscopic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as the iron oxides/oxyhydroxides are typically amorphous or microcrystalline coatings, not forming distinct cleavable minerals within the rock.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in weathered zones, soil profiles, gossans (oxidized caps over sulfide deposits), sedimentary rocks (especially sandstones and conglomerates), hydrothermal alteration zones, and as coatings on rocks exposed to surface weathering.

Key Facts

Hardness

Variable, depends on the host rock and the specific iron oxide/oxyhydroxide. Hematite (5-6), Goethite (5-5.5). The staining itself is often softer.

Specific Gravity

Variable, depends on the host rock and the amount/type of iron oxides. Hematite (5.26), Goethite (3.3-4.3).

Crystal System

Not applicable to the rock as a whole. Hematite is trigonal, Goethite is orthorhombic.

Color

Red, orange, yellow, brown

Luster

Dull, earthy, sometimes sub-metallic

Transparency

Opaque

Fracture

Variable, depends on the host rock. Iron oxide coatings are typically irregular or conchoidal.

Cleavage

None (for the staining itself)

Composition

Host rock minerals + secondary iron oxides/oxyhydroxides (e.g., Fe2O3, FeO(OH)·nH2O)

Physical characteristics

• Crystal Habit: Typically amorphous, cryptocrystalline, or microcrystalline coatings, cements, or disseminations. Rarely macroscopic crystals.
• Cleavage Type: Not applicable to the staining.
• Fracture Type: Variable, depends on the host rock. Iron oxide coatings often exhibit irregular or earthy fracture.
• Tenacity: Brittle (for the iron oxides themselves, but the overall rock tenacity depends on the host rock).
• Luster Type: Earthy to dull, occasionally sub-metallic.

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