How to identify Limonite/Goethite (Hydrous Iron Oxides)

To correctly identify Limonite/Goethite (Hydrous Iron Oxides) (Limonite/Goethite (hydrous iron oxides)), 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, or black. Often has a rusty appearance.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, submetallic, or silky (in fibrous varieties of goethite).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Earthy, porous, botryoidal (grape-like), stalactitic, mammillary, or concretionary. Can also be massive or fibrous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Goethite forms acicular (needle-like), fibrous, bladed, or tabular crystals, but is most commonly found in massive, botryoidal, reniform, or stalactitic aggregates. Limonite, being a mixture, does not have a distinct crystal form but rather an aggregate habit.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Goethite has perfect cleavage on {010}, but it is rarely observed in massive or fine-grained aggregates. Limonite exhibits no cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Weathering zones of iron-rich rocks and sulfide deposits, bog and swamp deposits, lateritic soils, and as cementing material in sandstones and conglomerates.

Key Facts

Hardness

Goethite: 5-5.5 on Mohs scale. Limonite: Varies, typically 4-5.5 (due to its aggregate nature and impurities).

Specific Gravity

Goethite: 3.3-4.3. Limonite: Varies, typically 2.7-4.3 (depending on porosity and composition).

Crystal System

Goethite: Orthorhombic. Limonite: Amorphous to cryptocrystalline (not a true mineral with a defined crystal system).

Color

Yellowish-brown, reddish-brown, dark brown, black.

Luster

Dull, earthy, submetallic, silky.

Transparency

Opaque.

Fracture

Uneven, conchoidal (for goethite), earthy.

Cleavage

Goethite: Perfect on {010}, but rarely seen. Limonite: None.

Composition

Goethite: Hydrous iron oxide (α-FeO(OH)). Limonite: A mixture of hydrous iron oxides, predominantly goethite, with variable amounts of water and impurities.

Physical characteristics

• Crystal Habit: Goethite: Acicular, fibrous, bladed, tabular, massive, botryoidal, reniform, stalactitic. Limonite: Massive, earthy, porous, concretionary, mammillary.
• Cleavage Type: Goethite: Perfect {010}. Limonite: None.
• Fracture Type: Goethite: Uneven to subconchoidal. Limonite: Earthy.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, submetallic, silky.

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