How to identify Goethite

To correctly identify Goethite (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

   Dark brown to black, sometimes yellowish-brown to reddish-brown. The botryoidal surfaces are often lustrous black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Adamantine to submetallic on fresh surfaces, often dull or earthy in massive forms. Botryoidal surfaces typically exhibit a submetallic to metallic luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Botryoidal (grape-like), mammillary (breast-like), reniform (kidney-shaped), stalactitic, or massive. Internally, it can be fibrous or radiating.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Orthorhombic, but well-formed crystals are rare. Typically occurs in massive, botryoidal, stalactitic, or fibrous aggregates. The botryoidal form is a macroscopic aggregate of minute, acicular crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect on {010}, but rarely observed due to its typical massive or fibrous habit. More commonly exhibits an uneven to subconchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in the oxidized zones of iron ore deposits, gossans, lateritic soils, bog iron deposits, and as a weathering product of iron-rich rocks and minerals. It forms in aqueous, oxidizing conditions.

Key Facts

Hardness

5.0 - 5.5 on the Mohs scale

Specific Gravity

3.3 - 4.3 (variable depending on porosity and impurities)

Crystal System

Orthorhombic

Color

Dark brown, black, yellowish-brown, reddish-brown

Luster

Adamantine to submetallic, dull, earthy

Transparency

Opaque

Fracture

Uneven to subconchoidal

Cleavage

Perfect on {010}, but rarely observed

Composition

Hydrated iron oxide (α-FeO(OH))

Physical characteristics

• Crystal Habit: Botryoidal, mammillary, reniform, stalactitic, fibrous, massive, earthy, acicular crystals (rarely distinct)
• Cleavage Type: Perfect on {010}, but typically not seen due to habit
• Fracture Type: Uneven to subconchoidal
• Tenacity: Brittle
• Luster Type: Submetallic to metallic on botryoidal surfaces; dull to earthy in massive forms

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