How to identify Plant Fossil in Shale

To correctly identify Plant Fossil in Shale (Fossilized plant impression in sedimentary rock (likely shale or mudstone)), 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

   The fossilized plant material is typically black, dark brown, or dark gray due to carbonization. The surrounding shale matrix can be various shades of gray, black, brown, or sometimes reddish-brown or greenish-gray, depending on its mineralogical composition and oxidation state.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   The carbonized plant material often exhibits a dull to sub-metallic or vitreous luster, especially if it's a thin carbon film. The shale matrix is typically dull to earthy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The shale matrix is fine-grained, smooth to slightly gritty, and often fissile (splits into thin layers). The fossil impression itself will follow the texture of the original plant material, often showing fine details like venation or stem patterns.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the fossil itself, as it is an organic remnant or impression. The shale matrix is composed of microscopic clay minerals, which are platy but not macroscopically crystalline.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   The shale matrix exhibits excellent basal cleavage (fissility), allowing it to split along bedding planes, often revealing the embedded fossils. The fossil itself does not have cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Low-energy depositional environments such as swamps, bogs, lakes, lagoons, deltas, and quiet marine shelves. These environments are characterized by fine sediment accumulation and often anoxic conditions that inhibit decomposition.

Key Facts

Hardness

Not applicable to the fossil itself. The shale matrix typically has a Mohs hardness of 2.5-4.

Specific Gravity

Not applicable to the fossil itself. The shale matrix typically ranges from 2.0 to 2.8 g/cm³.

Crystal System

Not applicable (fossil is organic/amorphous; shale is microcrystalline/amorphous clay minerals).

Color

Fossil: Black, dark brown. Shale: Gray, black, brown, sometimes red or green.

Luster

Fossil: Dull to sub-metallic. Shale: Dull, earthy.

Transparency

Opaque (both fossil and shale).

Fracture

Fossil: Irregular to conchoidal (if carbonized). Shale: Splintery to irregular, often platy due to fissility.

Cleavage

Fossil: None. Shale: Excellent basal cleavage (fissility).

Composition

Fossil: Primarily carbon (C), with trace elements. Shale: Predominantly clay minerals (e.g., illite, kaolinite, smectite), quartz (SiO2), feldspar, and organic matter.

Physical characteristics

• Crystal Habit: Not applicable to the fossil. Shale is microcrystalline to amorphous.
• Cleavage Type: Shale exhibits excellent basal cleavage (fissility) due to the alignment of platy clay minerals.
• Fracture Type: Irregular to splintery in shale; conchoidal to irregular in carbonized fossil material.
• Tenacity: Brittle (both fossil and shale).
• Luster Type: Dull to sub-metallic for the fossil; earthy to dull for the shale matrix.

Explore Plant Fossil in Shale