How to identify Goethite

To correctly identify Goethite (α-FeO(OH)), 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

   Yellowish-brown, reddish-brown, dark brown to black. Often appears rusty.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Adamantine to dull, silky (fibrous varieties), earthy (massive varieties).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Earthy, fibrous, botryoidal, stalactitic, reniform, or massive.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Orthorhombic. Crystals are typically prismatic, acicular (needle-like), or tabular, often striated. More commonly found in massive, botryoidal, reniform, or stalactitic aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect on {010}, but rarely observed in massive or fibrous forms.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Oxidation zones of iron ore deposits, gossans, lateritic soils, bogs, swamps, sedimentary iron formations, and as a weathering product of iron-bearing silicates and sulfides.

Key Facts

Hardness

5.0-5.5 (Mohs scale)

Specific Gravity

3.3-4.3 (variable depending on porosity and impurities)

Crystal System

Orthorhombic

Color

Yellowish-brown, reddish-brown, dark brown to black

Luster

Adamantine, submetallic, silky, dull, earthy

Transparency

Opaque

Fracture

Uneven to subconchoidal

Cleavage

Perfect on {010}, but rarely observed

Composition

Iron oxyhydroxide (α-FeO(OH))

Physical characteristics

• Crystal Habit: Prismatic, acicular, tabular, massive, botryoidal, reniform, stalactitic, fibrous, earthy
• Cleavage Type: Perfect on {010}, but often not apparent in massive forms
• Fracture Type: Uneven to subconchoidal
• Tenacity: Brittle
• Luster Type: Adamantine to dull, silky, earthy

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