Pyrite with secondary uranium minerals

Description

This refers to an association where the iron sulfide mineral pyrite (FeS2) is found in close proximity to, or intergrown with, secondary uranium minerals. Pyrite itself is a common sulfide mineral, often metallic yellow and cubic. The secondary uranium minerals, such as autunite (Ca(UO2)2(PO4)2·10-12H2O) and carnotite (K2(UO2)2(VO4)2·3H2O), are typically brightly colored (yellow, green, orange) and often form crusts, coatings, or small crystals on the host rock or other minerals, including pyrite. This association indicates a geological history involving primary uranium mineralization, followed by oxidation and remobilization of uranium, leading to the formation of these secondary phases.

How to Identify

Color

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.

Luster

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.

Texture

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.

Crystal Form

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.

Cleavage

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

Geological Environment

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).

Quick Check

• Color: Metallic brass-yellow (pyrite) combined with bright yellow, green, or orange (secondary uranium minerals).
• Luster: Metallic (pyrite) to vitreous, pearly, or earthy (secondary uranium minerals).
• Streak: Greenish-black (pyrite); secondary uranium minerals typically have a pale yellow or colorless streak, but are often too soft or powdery to produce a distinct streak on a streak plate.

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.

Formation

Pyrite (FeS2) forms in a wide range of geological environments, typically under reducing conditions. When pyrite is associated with primary uranium minerals (e.g., uraninite, pitchblende) and subsequently exposed to oxidizing conditions (e.g., weathering, groundwater circulation), the primary uranium minerals can alter to form a suite of secondary uranium minerals. These secondary minerals often precipitate on or near the pyrite, or within the same host rock, due to changes in pH, Eh, and the availability of other elements (e.g., phosphates, vanadates, carbonates). Pyrite itself can also act as a reductant, influencing the precipitation of some uranium minerals.

Usage

This assemblage is primarily of scientific interest for understanding uranium mineralization, alteration processes, and environmental geochemistry. It serves as an indicator of potential uranium deposits. Due to the radioactivity, it is not used for ornamental or industrial purposes in this form.

Age Distribution

Can occur in geological formations of various ages, from Precambrian to Cenozoic, wherever primary uranium mineralization and subsequent alteration have taken place.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Pyrite: Sulfide. Secondary uranium minerals: Phosphates, Vanadates, Silicates, Carbonates (depending on the specific mineral).

Group

Pyrite: Pyrite Group. Secondary uranium minerals: Autunite Group, Carnotite Group, etc.

Crystal System

Pyrite: Isometric. Secondary uranium minerals: Tetragonal (autunite, torbernite), Monoclinic (carnotite, uranophane).

Chemical Formula

FeS2 (Pyrite) with various formulas for secondary uranium minerals, e.g., Ca(UO2)2(PO4)2·10-12H2O (Autunite), K2(UO2)2(VO4)2·3H2O (Carnotite).

Composition

Pyrite: Iron (Fe), Sulfur (S). Secondary uranium minerals: Uranium (U), Oxygen (O), and various other elements such as Calcium (Ca), Potassium (K), Phosphorus (P), Vanadium (V), Silicon (Si), Carbon (C), and Hydrogen (H) in the form of water.

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