How to identify Brachiopod Fossil

To correctly identify Brachiopod Fossil (Brachiopoda (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

   Variable, depending on the matrix rock and diagenetic alteration. Often shades of grey, brown, tan, black, or white. Original shell material, if preserved, is typically white or off-white.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy if preserved in a matrix; vitreous to pearly if original shell material is well-preserved and clean.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to ribbed, often with concentric growth lines. The texture reflects the external morphology of the original shell.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the fossil itself, as it is an organic remnant. The shell material, if preserved, is microcrystalline calcite or aragonite, or cryptocrystalline apatite.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the fossil as a whole. The constituent mineral (calcite) would exhibit perfect rhombohedral cleavage, but this is rarely observable in fossilized shells.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine sedimentary environments, particularly shallow shelf seas, reefs, and deeper water settings where sedimentation rates allowed for preservation. Commonly found in limestones, shales, sandstones, and siltstones.

Key Facts

Hardness

Not applicable to the fossil as a whole. The shell material (calcite) has a Mohs hardness of 3.0. Phosphate shells (apatite) are harder, around 5.0.

Specific Gravity

Variable, depending on preservation and infilling minerals. Original calcite shells are around 2.71 g/cm³.

Crystal System

Not applicable to the fossil. The shell material (calcite) is trigonal.

Color

Variable, often matching the host rock or white/off-white if original shell is preserved.

Luster

Dull to earthy; vitreous to pearly if well-preserved.

Transparency

Opaque to translucent (if original shell material is thin and well-preserved).

Fracture

Conchoidal to uneven (if original shell material is preserved).

Cleavage

Not applicable to the fossil. Calcite has perfect rhombohedral cleavage.

Composition

Primarily calcium carbonate (CaCO3) in the form of calcite or aragonite, or calcium phosphate (Ca5(PO4)3(OH,F,Cl)) for some groups (e.g., Linguliformea). The fossil itself is often composed of the infilling or replacing minerals.

Physical characteristics

• Crystal Habit: Not applicable to the fossil. The shell structure is biomineralized, not a single crystal.
• Cleavage Type: Not applicable to the fossil. Calcite has perfect rhombohedral cleavage.
• Fracture Type: Conchoidal to uneven (for shell material).
• Tenacity: Brittle.
• Luster Type: Dull, earthy, vitreous, pearly.

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