How to identify Trilobite Fossil

To correctly identify Trilobite Fossil (Phylum Arthropoda, Class Trilobita), 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

   Variable, depending on the host rock and replacement minerals. Commonly grey, black, brown, tan, or reddish. Pyritized specimens can be golden or brassy.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy if preserved as an impression in shale or sandstone. Can be vitreous (glassy) if replaced by silica, or metallic if pyritized. Calcite-replaced specimens may have a dull to sub-vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The surface texture often reflects the original exoskeleton, showing segmentation, tubercles, or spines. The surrounding rock matrix will have its own texture (e.g., fine-grained shale, gritty sandstone).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not a mineral, but a fossil. The preserved form is the morphology of the trilobite exoskeleton, characterized by its distinctive three-lobed, segmented structure. Complete specimens show a cephalon, segmented thorax, and pygidium.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the fossil itself, as it is an organic remnant or replacement. The host rock may exhibit cleavage (e.g., shales).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine sedimentary environments, typically shallow to deep marine shelves, often found in shales, limestones, and sandstones that formed in ancient seas. They are indicative of marine depositional settings.

Key Facts

Hardness

Variable, depending on the replacement mineral and host rock. Calcite (3), Quartz (7), Pyrite (6-6.5). The fossil itself is not a mineral with a fixed hardness.

Specific Gravity

Variable, depending on the replacement mineral and host rock. Calcite (2.7), Quartz (2.65), Pyrite (4.9-5.2).

Crystal System

Not applicable (fossil, not a mineral)

Color

Variable, often matching or contrasting with the host rock.

Luster

Variable, from dull to metallic, depending on preservation.

Transparency

Opaque

Fracture

Not applicable to the fossil itself; the host rock will exhibit fracture (e.g., conchoidal in chert, irregular in shale).

Cleavage

Not applicable to the fossil itself; the host rock may exhibit cleavage.

Composition

Primarily calcium carbonate (original exoskeleton, often replaced by calcite or silica), or iron sulfide (pyrite), or preserved as carbonaceous film, or as molds/casts within sedimentary rock.

Physical characteristics

• Crystal Habit: Not applicable (fossil)
• Cleavage Type: Not applicable (fossil)
• Fracture Type: Not applicable (fossil)
• Tenacity: Brittle (depending on the replacement mineral and host rock)
• Luster Type: Variable (dull, earthy, vitreous, metallic)

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