How to identify Graptolitic Shale

To correctly identify Graptolitic Shale (Shale (sedimentary rock) with Graptolithina (fossil)), 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 to black, sometimes greenish-gray or brownish-black. The graptolite fossils themselves are usually black or dark brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy on fresh surfaces; may appear slightly waxy or greasy if rich in organic matter. Graptolite fossils often have a slightly metallic or carbonaceous sheen.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Very fine-grained, smooth to the touch. Exhibits fissility, meaning it readily splits along bedding planes into thin, platy fragments. Graptolite fossils are preserved as flattened impressions on these planes.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the rock matrix, as it is composed of microscopic clay minerals and detrital grains. Graptolites are fossilized colonial organisms, not crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   The rock exhibits fissility, which is a type of parting along bedding planes, not true mineral cleavage. Graptolites are preserved on these planes.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Deep marine basins, continental slopes, and anoxic shelf environments where fine-grained sediments accumulate slowly and organic matter is preserved. Often associated with turbidite sequences or quiet, oxygen-depleted waters.

Key Facts

Hardness

2.5-3.0 (Mohs scale for the shale matrix, due to clay minerals and quartz)

Specific Gravity

2.0-2.8 (variable, depending on composition and organic content)

Crystal System

Not applicable (rock composed of microscopic minerals; graptolites are fossils)

Color

Dark gray, black, greenish-gray (shale); black (graptolites)

Luster

Dull to earthy (shale); submetallic to dull (graptolites)

Transparency

Opaque

Fracture

Splintery to conchoidal (shale), 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, organic matter (kerogen), and carbonized graptolite fossils (chitinous composition converted to carbon)

Physical characteristics

• Crystal Habit: Not applicable for the rock; graptolites are colonial organisms with distinct rhabdosome (colony) forms (e.g., uniserial, biserial, multiserial, dendroid).
• Cleavage Type: Fissility (splits along bedding planes)
• Fracture Type: Irregular to splintery, sometimes subconchoidal
• Tenacity: Brittle
• Luster Type: Dull to earthy for the shale matrix; graptolite fossils often have a submetallic or carbonaceous sheen.

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