How to identify Rainbow Hematite

To correctly identify Rainbow Hematite (Hematite with iridescent tarnish (Fe2O3)), 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

   Metallic gray to black with an iridescent surface displaying a spectrum of colors (blue, green, yellow, orange, red, purple). The underlying hematite is typically dark.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be botryoidal, reniform, massive, or platy. The iridescent layer is a surface phenomenon and does not alter the underlying texture significantly.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically found as botryoidal (kidney-shaped) or reniform (kidney-like) masses, sometimes as tabular crystals or micaceous flakes. The iridescent coating conforms to these forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, though parting may be observed on the basal pinacoid {0001} due to twinning or lamellar structure.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in various hematite-forming environments, including hydrothermal veins, contact metamorphic zones, and sedimentary deposits. The iridescent tarnish is a secondary alteration, often occurring in weathering environments or low-temperature hydrothermal settings where thin films of other minerals can precipitate on the hematite surface.

Key Facts

Hardness

5.5 - 6.5 on the Mohs scale (for the underlying hematite)

Specific Gravity

5.26 (for pure hematite)

Crystal System

Trigonal

Color

Metallic gray to black with iridescent surface colors (blue, green, yellow, orange, red, purple)

Luster

Metallic to submetallic, iridescent

Transparency

Opaque

Fracture

Uneven to subconchoidal

Cleavage

None (parting on {0001} and {1011} may be present)

Composition

Iron(III) oxide (Fe2O3) with a thin surface layer causing iridescence

Physical characteristics

• Crystal Habit: Massive, botryoidal, reniform, tabular, micaceous (specular hematite), stalactitic. The iridescent layer coats these forms.
• Cleavage Type: Absent. Parting may occur on {0001} and {1011}.
• Fracture Type: Uneven to subconchoidal
• Tenacity: Brittle
• Luster Type: Metallic to submetallic, with an iridescent surface sheen

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