How to identify Goethite with Limonite

To correctly identify Goethite with Limonite (FeO(OH) with FeO(OH)·nH2O), 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 to dark brown, blackish-brown, or reddish-brown. Limonite is typically more yellowish-brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Goethite can be adamantine, submetallic, silky (fibrous varieties), or dull/earthy. Limonite is typically dull and earthy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Goethite can be fibrous, botryoidal, stalactitic, reniform, or massive. Limonite is typically earthy, porous, concretionary, mammillary, or stalactitic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Goethite forms prismatic, acicular, or tabular crystals, often radiating or fibrous. Limonite is typically amorphous or cryptocrystalline, lacking distinct crystal forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Goethite exhibits perfect cleavage on {010} and good cleavage on {100}. Limonite, being an aggregate, does not exhibit true cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in the oxidized zones of iron ore deposits, gossans, bogs, swamps, and as a weathering product of iron-rich rocks and minerals. Common in sedimentary environments where iron-rich waters precipitate.

Key Facts

Hardness

Goethite: 5-5.5 (Mohs); Limonite: 1-5.5 (variable, often softer and earthy)

Specific Gravity

Goethite: 3.3-4.3; Limonite: 2.7-4.3 (variable)

Crystal System

Goethite: Orthorhombic; Limonite: Amorphous to cryptocrystalline (no defined crystal system)

Color

Yellowish-brown, brown, blackish-brown, reddish-brown

Luster

Adamantine, submetallic, silky (fibrous), dull, earthy

Transparency

Opaque

Fracture

Goethite: Uneven to subconchoidal; Limonite: Earthy, conchoidal

Cleavage

Goethite: Perfect on {010}, good on {100}; Limonite: None (aggregate)

Composition

Goethite: Iron oxyhydroxide (FeO(OH)); Limonite: Hydrous iron oxides, predominantly goethite, with variable water content (FeO(OH)·nH2O)

Physical characteristics

• Crystal Habit: Goethite: Prismatic, acicular, tabular, fibrous, radiating, botryoidal, stalactitic, reniform, massive. Limonite: Earthy, porous, concretionary, mammillary, stalactitic, massive.
• Cleavage Type: Goethite: Perfect {010}, good {100}; Limonite: None
• Fracture Type: Goethite: Uneven to subconchoidal; Limonite: Earthy, conchoidal
• Tenacity: Goethite: Brittle; Limonite: Friable to brittle
• Luster Type: Adamantine, submetallic, silky, dull, earthy

Explore Goethite with Limonite