How to identify Brachiopod Fossil

To correctly identify Brachiopod Fossil (Phylum Brachiopoda), 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 original shell composition and subsequent diagenesis. Often white, gray, brown, black, or reddish-brown. If silicified, can be translucent or glassy.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy if preserved as original shell material or mold/cast. Vitreous to waxy if silicified. Pearly if original nacreous layers are preserved (rare).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, ribbed, or spiny, reflecting the external morphology of the shell. Internal molds may show muscle scars or vascular markings.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the organism itself. The shell material is typically microcrystalline calcite or apatite. Fossils are preserved as original shell material, internal/external molds, or mineral replacements.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the fossil as a whole. The constituent minerals (calcite, apatite, quartz) exhibit their characteristic cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine sedimentary rocks, particularly limestones, shales, and sandstones, formed in shallow to deep marine environments. Common in ancient continental shelf deposits.

Key Facts

Hardness

Variable. Original calcite shells are 3 on Mohs scale. Original apatite shells are 5. If silicified, 7. If pyritized, 6-6.5.

Specific Gravity

Variable. Original calcite shells ~2.7. Original apatite shells ~3.1. If silicified, ~2.65. If pyritized, ~5.0.

Crystal System

Not applicable to the organism. Shell material is typically microcrystalline, often orthorhombic (calcite) or hexagonal (apatite).

Color

Variable, as described above.

Luster

Variable, as described above.

Transparency

Opaque to translucent (especially if silicified).

Fracture

Conchoidal to uneven, depending on the preservation and mineral composition.

Cleavage

Not applicable to the fossil form; refers to the constituent minerals.

Composition

Primarily calcium carbonate (CaCO3) in articulate brachiopods, or calcium phosphate (Ca5(PO4)3(OH,F,Cl)) in inarticulate brachiopods. Can be replaced by silica (SiO2), pyrite (FeS2), or other minerals.

Physical characteristics

• Crystal Habit: Not applicable to the fossil as a whole. Shells are biogenic structures.
• Cleavage Type: Not applicable to the fossil as a whole. Calcite has perfect rhombohedral cleavage; apatite has poor basal cleavage.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, vitreous, waxy, or pearly.

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