How to identify Iridescent Goethite

To correctly identify Iridescent Goethite (Goethite (α-FeOOH)), 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

   Base color is dark brown to black, but the surface exhibits strong metallic iridescence with colors like blue, green, gold, purple, and red.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Submetallic to metallic, with a distinct iridescent sheen on the surface.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often botryoidal, stalactitic, reniform, or massive. Surfaces can be smooth or mammillary.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically forms in radiating fibrous aggregates, botryoidal, stalactitic, or reniform masses. Individual crystals are rare and usually acicular or prismatic.

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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in the oxidized zones of iron ore deposits, gossans, lateritic soils, and as a weathering product of iron-rich minerals. It can also form in bog iron deposits and hydrothermal veins.

Key Facts

Hardness

5 - 5.5 on the Mohs scale.

Specific Gravity

3.3 - 4.3, typically around 4.0.

Crystal System

Orthorhombic.

Color

Dark brown to black, with iridescent surface colors.

Luster

Submetallic to metallic, with strong iridescence.

Transparency

Opaque.

Fracture

Uneven to splintery.

Cleavage

Perfect on {010}, but rarely observed.

Composition

Hydrous iron oxide (α-FeOOH).

Physical characteristics

• Crystal Habit: Typically botryoidal, stalactitic, reniform, massive, fibrous, or acicular. Rarely forms distinct prismatic crystals.
• Cleavage Type: Perfect on one direction ({010}), but often obscured by its common habits.
• Fracture Type: Uneven to splintery.
• Tenacity: Brittle.
• Luster Type: Submetallic to metallic, with a characteristic iridescent sheen.

Explore Iridescent Goethite