How to identify Fossiliferous Shale

To correctly identify Fossiliferous Shale (Shale with plant fossil impressions), 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

   Typically dark gray, black, brown, or greenish-gray, often indicative of organic content and redox conditions during deposition. Lighter colors (light gray, tan) can occur if organic content is low.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes slightly waxy if rich in certain clay minerals or organic matter.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Very fine-grained, smooth to slightly gritty. Exhibits fissility, meaning it splits readily along bedding planes into thin, platy fragments. The plant fossils themselves will have a distinct texture, often appearing as flattened impressions or carbonized films.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral grains are microscopic and anhedral. Plant fossils retain their original macroscopic forms, albeit flattened.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Shale exhibits fissility, which is a parting along bedding planes, not true mineral cleavage. This is due to the parallel alignment of platy clay minerals during compaction. The rock breaks into thin, platy fragments.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Low-energy depositional environments such as lakes, swamps, lagoons, river floodplains, and quiet marine shelves. These settings allow for the accumulation of fine sediments and the preservation of delicate plant material due to rapid burial and often anoxic conditions.

Key Facts

Hardness

2.5-4 on Mohs scale (for the rock matrix, individual minerals vary)

Specific Gravity

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

Crystal System

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

Color

Dark gray, black, brown, greenish-gray, sometimes lighter shades.

Luster

Dull, earthy, sometimes slightly waxy.

Transparency

Opaque.

Fracture

Irregular to conchoidal (within individual layers), but typically breaks along fissile planes.

Cleavage

Exhibits fissility (parting along bedding planes), not true mineral cleavage.

Composition

Predominantly clay minerals (e.g., illite, kaolinite, smectite), quartz silt, and varying amounts of organic matter (kerogen, carbonized plant remains), feldspar, micas, and accessory minerals like pyrite, calcite, and iron oxides.

Physical characteristics

• Crystal Habit: Not applicable for a rock. Constituent clay minerals are platy and microscopic.
• Cleavage Type: Fissility (parting along bedding planes).
• Fracture Type: Irregular to sub-conchoidal within layers, but primarily breaks along fissile planes.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

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