How to identify Fossiliferous Shale

To correctly identify Fossiliferous Shale (Shale (sedimentary rock) with fossilized organism), 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

   Highly variable, commonly gray, black, green, red, or brown, depending on the organic content, iron oxides, and other mineral impurities. Black shales are typically rich in organic matter.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes slightly waxy if rich in organic matter.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Very fine-grained (clastic), smooth to the touch. Exhibits fissility, meaning it splits readily along bedding planes into thin, platy fragments. The fossils themselves will have distinct textures depending on their original composition and preservation.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral grains are microscopic and anhedral. Fossils retain their original biological forms, which can be highly varied.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not true mineral cleavage, but rather a rock property called fissility, which is the tendency to split into thin, parallel layers along bedding planes. This is distinct from the slaty cleavage of metamorphic slate.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Low-energy depositional environments such as deep marine basins, continental shelves, lakes, lagoons, swamps, and floodplains. These environments facilitate the settling of fine particles and the burial and preservation of organic remains.

Key Facts

Hardness

2.5-3.5 (Mohs scale for constituent minerals, but rock as a whole is friable)

Specific Gravity

2.0-2.8 g/cm³ (variable, depends on composition and compaction)

Crystal System

Not applicable for a rock; constituent minerals are typically monoclinic (clays) or hexagonal (quartz).

Color

Highly variable, commonly gray, black, green, red, or brown.

Luster

Dull to earthy.

Transparency

Opaque.

Fracture

Irregular to conchoidal (for individual grains), but typically breaks along fissile planes.

Cleavage

Exhibits fissility (rock property, not mineral cleavage).

Composition

Predominantly clay minerals (e.g., illite, kaolinite, smectite), silt-sized quartz, feldspar, micas, organic matter, and various accessory minerals. The fossil component is biogenic.

Physical characteristics

• Crystal Habit: Not applicable for a rock; constituent clay minerals are platy, quartz is anhedral.
• Cleavage Type: Fissility (rock property, splitting along bedding planes).
• Fracture Type: Irregular to splintery along non-fissile directions.
• Tenacity: Brittle, friable.
• Luster Type: Dull to earthy.

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