How to identify Iron-stained rock

To correctly identify Iron-stained rock (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

   Predominantly red, reddish-brown, orange, yellow, or brown. The specific hue depends on the dominant iron oxide: hematite (red), goethite (yellow-brown), or limonite (various shades of brown-yellow).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Typically dull to earthy, but can be sub-metallic if hematite is well-crystallized or specular. Often masked by the host rock's luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable, as it's a staining phenomenon. The texture will be that of the host rock (e.g., granular for sandstone, crystalline for granite). The iron oxides may form a powdery coating, a cement, or a pervasive impregnation.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Iron oxides often occur as microcrystalline aggregates, earthy masses, or botryoidal/reniform habits (e.g., goethite, hematite). Individual crystals are usually too small to be seen without magnification.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed for the iron oxide staining itself, as it's typically amorphous or microcrystalline. The host rock may exhibit its characteristic cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in weathered zones of all rock types, particularly in humid and oxidizing environments. Found in soils, saprolites, gossans (weathered sulfide deposits), sedimentary rocks (especially sandstones and shales), and altered igneous/metamorphic rocks.

Key Facts

Hardness

Variable, depending on the specific iron oxide and its form. Hematite: 5-6.5; Goethite: 5-5.5 (Mohs scale). Often appears softer due to earthy habit.

Specific Gravity

Variable. Hematite: 5.26; Goethite: 3.3-4.3. The overall specific gravity of the stained rock will be influenced by the host rock.

Crystal System

Hematite: Trigonal; Goethite: Orthorhombic. Often cryptocrystalline or amorphous in staining.

Color

Red, reddish-brown, orange, yellow, brown

Luster

Dull, earthy, sub-metallic

Transparency

Opaque

Fracture

Conchoidal to uneven (for individual minerals), but often earthy or friable in stained rocks.

Cleavage

None (for iron oxides in staining), but host rock may exhibit cleavage.

Composition

Iron oxides and oxyhydroxides (Fe2O3, FeO(OH)) coating or impregnating a host rock of variable composition.

Physical characteristics

• Crystal Habit: Earthy, massive, botryoidal, reniform, stalactitic, or as coatings and disseminations.
• Cleavage Type: Not applicable for the staining itself; host rock properties dominate.
• Fracture Type: Earthy, uneven, conchoidal (for individual minerals).
• Tenacity: Brittle to friable, depending on the form of the iron oxide.
• Luster Type: Earthy, dull, sub-metallic.

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