How to identify Ironstone Concretion with Pyrite

To correctly identify Ironstone Concretion with Pyrite (Ironstone concretion with Pyrite (FeS2)), 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

   Exterior typically reddish-brown, dark brown, or gray due to iron oxides/hydroxides or siderite. Pyrite within is brassy yellow to pale brass-yellow.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pyrite exhibits a metallic luster. The surrounding ironstone matrix can be dull, earthy, or sub-metallic.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Dense, compact, often fine-grained. Pyrite crystals can be euhedral (well-formed cubes, octahedra, pyritohedra) or anhedral (irregular grains). The concretion surface may be smooth or botryoidal.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite commonly forms cubes, octahedra, pyritohedra, or combinations thereof. It can also be massive, granular, or radiating. Marcasite, a polymorph of FeS2, forms orthorhombic crystals, often radiating or coxcomb aggregates, and can be present alongside or instead of pyrite.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Pyrite has indistinct cleavage, typically parting along cubic planes. The ironstone matrix generally lacks distinct cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in marine shales, mudstones, siltstones, and coal seams, particularly those deposited in anoxic or dysaerobic conditions. Also found in some sandstones and limestones.

Key Facts

Hardness

6-6.5 (Pyrite on Mohs scale); the overall concretion hardness varies depending on matrix composition, but is generally harder than the host rock.

Specific Gravity

4.95-5.03 (Pyrite); the overall concretion specific gravity will be lower, typically 2.7-3.5, depending on the proportion of pyrite and matrix minerals.

Crystal System

Isometric (Pyrite); Orthorhombic (Marcasite, if present). The concretion itself is an aggregate.

Color

Brassy yellow to pale brass-yellow (pyrite); exterior of concretion often dark brown, reddish-brown, or gray.

Luster

Metallic (pyrite); dull, earthy, or sub-metallic (matrix).

Transparency

Opaque (pyrite and most ironstone minerals).

Fracture

Conchoidal to uneven (pyrite); uneven to splintery (matrix).

Cleavage

Indistinct (pyrite); generally absent in the concretion matrix.

Composition

Iron disulfide (FeS2) for pyrite, within a matrix primarily composed of iron carbonates (siderite), iron oxides/hydroxides (goethite, hematite), clay minerals, quartz, and organic matter.

Physical characteristics

• Crystal Habit: Pyrite: Cubes, octahedra, pyritohedra, massive, granular, radiating. Concretion: Spherical, ovoid, discoidal, irregular, often with a smooth or botryoidal surface.
• Cleavage Type: Pyrite: Indistinct, parting on {100}. Concretion matrix: None.
• Fracture Type: Pyrite: Conchoidal to uneven. Concretion matrix: Uneven to splintery.
• Tenacity: Brittle (both pyrite and the concretion).
• Luster Type: Metallic (pyrite); dull, earthy, or sub-metallic (matrix).

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