How to identify Shale with plant fossil

To correctly identify Shale with plant fossil (Shale (sedimentary rock) with plant fossil (paleobotanical remains)), 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 reddish-brown, depending on the organic content and iron oxides. The plant fossils themselves are usually black or dark brown carbonaceous films.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the shale matrix; the carbonaceous plant fossils may exhibit a slightly sub-metallic or dull luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Very fine-grained, smooth to slightly gritty feel. Characterized by fissility, meaning it splits readily along bedding planes. The plant fossils are often preserved as flattened impressions or thin carbon films on these planes.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the rock as a whole. Individual clay minerals are microscopic and platy. Plant fossils retain the morphology of the original plant parts.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Exhibits fissility, a type of parting along bedding planes, rather than true mineral cleavage. The rock splits into thin, platy fragments.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Low-energy depositional environments such as swamps, bogs, deltas, floodplains, lagoons, and quiet shallow marine basins. Often found in association with coal seams, sandstones, and limestones.

Key Facts

Hardness

2.5-3.5 on Mohs scale (for the shale matrix). The carbonaceous fossil material is softer.

Specific Gravity

2.0-2.8 (variable depending on composition and compaction).

Crystal System

Not applicable (rock composed of microscopic, often disordered, clay minerals).

Color

Dark gray, black, brown, reddish-brown. Plant fossils are typically black.

Luster

Dull to earthy (shale); sub-metallic to dull (fossils).

Transparency

Opaque.

Fracture

Irregular to conchoidal (across bedding planes); fissile (along bedding planes).

Cleavage

No true mineral cleavage; exhibits fissility (rock property).

Composition

Predominantly clay minerals (e.g., illite, kaolinite, smectite), quartz silt, and organic matter (including carbonized plant remains). May contain minor feldspar, micas, and iron oxides.

Physical characteristics

• Crystal Habit: Not applicable for the rock; clay minerals are platy and microscopic.
• Cleavage Type: Fissility (a rock property, not mineral cleavage), splitting into thin, parallel layers.
• Fracture Type: Irregular to sub-conchoidal when broken across bedding planes.
• Tenacity: Brittle.
• Luster Type: Dull to earthy for the shale matrix; the carbonaceous plant fossils may have a slightly sub-metallic or dull luster.

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