How to identify Iron Oxide

To correctly identify Iron Oxide (Iron Oxide (e.g., Goethite, Limonite, Hematite)), 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

   Hematite: Steel-gray to reddish-brown. Goethite/Limonite: Yellowish-brown to dark brown, sometimes reddish-brown. The streak test is often more diagnostic than body color.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Hematite: Metallic to dull earth. Goethite/Limonite: Earthy, dull, silky (in fibrous varieties).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Hematite: Can be massive, botryoidal, reniform, micaceous (specular hematite), or oolitic. Goethite/Limonite: Typically massive, botryoidal, stalactitic, or earthy.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Hematite: Trigonal system, often forming tabular, rhombohedral, or platy crystals. Goethite: Orthorhombic system, typically acicular, prismatic, or massive. Limonite: Amorphous or cryptocrystalline, rarely showing distinct crystal forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Hematite: No true cleavage, but parting along rhombohedral planes can occur. Goethite: Perfect {010} cleavage, but often not observed in massive forms. Limonite: None.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in a wide range of environments. Hematite is common in banded iron formations, hydrothermal veins, and as a weathering product. Goethite and Limonite are characteristic of gossans, bog iron ores, and as secondary minerals in soils and sedimentary rocks, forming from the oxidation of other iron-bearing minerals.

Key Facts

Hardness

Hematite: 5-6.5. Goethite: 5-5.5. Limonite: 4-5.5 (variable due to amorphous nature).

Specific Gravity

Hematite: 5.26. Goethite: 3.3-4.3. Limonite: 2.7-4.3 (variable).

Crystal System

Hematite: Trigonal. Goethite: Orthorhombic. Limonite: Amorphous/Cryptocrystalline.

Color

Steel-gray, reddish-brown, yellowish-brown, dark brown.

Luster

Metallic, submetallic, dull, earthy, silky.

Transparency

Opaque.

Fracture

Hematite: Uneven to subconchoidal. Goethite: Uneven. Limonite: Earthy.

Cleavage

Hematite: None (parting present). Goethite: Perfect {010}. Limonite: None.

Composition

Iron and Oxygen (with varying hydration for goethite/limonite).

Physical characteristics

• Crystal Habit: Hematite: Massive, botryoidal, reniform, micaceous, tabular, rhombohedral. Goethite: Acicular, prismatic, massive, botryoidal, stalactitic. Limonite: Massive, earthy, botryoidal, stalactitic.
• Cleavage Type: Hematite: None (parting). Goethite: Perfect {010}. Limonite: None.
• Fracture Type: Uneven, subconchoidal, earthy.
• Tenacity: Brittle.
• Luster Type: Metallic, submetallic, dull, earthy, silky.

Explore Iron Oxide