How to identify Pyrite with secondary uranium minerals

To correctly identify Pyrite with secondary uranium minerals (FeS2 with secondary uranium minerals (e.g., autunite, carnotite)), 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

   Pyrite is typically pale brass-yellow to golden. Secondary uranium minerals exhibit a range of bright colors: autunite is typically lemon-yellow to greenish-yellow; carnotite is canary-yellow to greenish-yellow; torbernite is emerald-green; uranophane is lemon-yellow. The overall appearance will be a combination of these colors.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pyrite has a metallic luster. Secondary uranium minerals can have a vitreous, pearly, or earthy luster, depending on the specific mineral and its crystal habit.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Pyrite can be massive, granular, or form distinct crystals (cubes, pyritohedrons). Secondary uranium minerals often form powdery coatings, earthy masses, micaceous flakes, or small tabular crystals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite commonly forms euhedral to subhedral cubes, pyritohedrons, or octahedra. Secondary uranium minerals vary: autunite forms tabular to platy crystals; carnotite is typically earthy or powdery; torbernite forms square tabular crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Pyrite has indistinct cleavage. Secondary uranium minerals often exhibit perfect cleavage (e.g., autunite, torbernite) or no distinct cleavage (e.g., carnotite).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in oxidized zones of uranium deposits, often associated with fault zones, shear zones, or permeable host rocks where primary uranium minerals have been altered by circulating oxygenated groundwater. Can occur in sedimentary rocks (e.g., sandstones), metamorphic rocks, and hydrothermal veins.

Key Facts

Hardness

Pyrite: 6-6.5 (Mohs). Secondary uranium minerals: typically much softer, ranging from 2-3 (e.g., autunite, carnotite).

Specific Gravity

Pyrite: 4.95-5.10. Secondary uranium minerals: generally lower, e.g., autunite ~3.1-3.2, carnotite ~4.0-4.1.

Crystal System

Pyrite: Isometric. Secondary uranium minerals: Autunite (Tetragonal), Carnotite (Monoclinic), Torbernite (Tetragonal), Uranophane (Monoclinic).

Color

Pyrite: Pale brass-yellow. Secondary uranium minerals: Bright yellow, greenish-yellow, green, orange.

Luster

Pyrite: Metallic. Secondary uranium minerals: Vitreous, pearly, earthy.

Transparency

Pyrite: Opaque. Secondary uranium minerals: Transparent to translucent (crystals), opaque (earthy masses).

Fracture

Pyrite: Uneven to conchoidal. Secondary uranium minerals: Uneven, splintery, or micaceous.

Cleavage

Pyrite: Indistinct. Secondary uranium minerals: Perfect (e.g., autunite, torbernite) or none (e.g., carnotite).

Composition

Pyrite: Iron disulfide (FeS2). Secondary uranium minerals: Hydrated uranium phosphates (e.g., autunite, torbernite), hydrated uranium vanadates (e.g., carnotite), hydrated uranium silicates (e.g., uranophane), and others. All contain uranium (U).

Physical characteristics

• Crystal Habit: Pyrite: Cubes, pyritohedrons, octahedra, massive, granular. Secondary uranium minerals: Tabular, platy, micaceous, earthy, powdery, acicular.
• Cleavage Type: Pyrite: Indistinct. Secondary uranium minerals: Perfect basal (e.g., autunite, torbernite) or none.
• Fracture Type: Pyrite: Uneven to conchoidal. Secondary uranium minerals: Uneven, splintery, or micaceous.
• Tenacity: Pyrite: Brittle. Secondary uranium minerals: Brittle, sectile (some micaceous forms).
• Luster Type: Pyrite: Metallic. Secondary uranium minerals: Vitreous, pearly, earthy.

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