How to identify Iron-stained rock

To correctly identify Iron-stained rock (Rock with iron oxide staining), 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 red, orange, yellow, brown, or reddish-brown. The specific hue depends on the iron oxide mineral present (e.g., hematite for red, goethite for yellow/brown).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes sub-metallic if the iron oxide is more crystalline (e.g., specular hematite).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable, depending on the host rock. The staining itself can be powdery, earthy, or a thin, hard coating.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Iron oxides responsible for staining are often cryptocrystalline, amorphous, or fine-grained, lacking distinct crystal forms visible to the naked eye. They may form botryoidal, reniform, or stalactitic masses in cavities.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None for the staining material itself, as it is typically an aggregate or coating. The host rock may exhibit its characteristic cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Ubiquitous in surficial environments, weathering zones, hydrothermal alteration zones, and sedimentary environments. Common in outcrops exposed to air and water, fault zones, and areas with sulfide mineralization.

Key Facts

Hardness

Variable, depending on the specific iron oxide and its form. Hematite (5-6.5), Goethite (5-5.5). The staining itself is often soft and powdery (1-2).

Specific Gravity

Variable, depending on the specific iron oxide. Hematite (4.9-5.3), Goethite (3.3-4.3).

Crystal System

Variable, depending on the specific iron oxide. Hematite (Trigonal), Goethite (Orthorhombic). Limonite is amorphous or cryptocrystalline.

Color

Red, orange, yellow, brown, reddish-brown

Luster

Dull, earthy, sometimes sub-metallic

Transparency

Opaque

Fracture

Earthy, conchoidal (for more massive forms of iron oxides)

Cleavage

None (for the staining material itself)

Composition

Iron oxides and hydroxides (e.g., Fe2O3, FeO(OH))

Physical characteristics

• Crystal Habit: Typically massive, earthy, botryoidal, reniform, or as thin coatings and impregnations. Individual crystals are usually microscopic.
• Cleavage Type: Not applicable for the staining material itself, as it is often amorphous or microcrystalline.
• Fracture Type: Earthy to uneven, sometimes conchoidal in more compact forms.
• Tenacity: Brittle to friable (powdery).
• Luster Type: Dull, earthy, sometimes sub-metallic.

Explore Iron-stained rock