How to identify Brachiopod Fossil

To correctly identify Brachiopod Fossil (Brachiopoda (fossilized)), 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

   Highly variable, depending on the original shell composition and the mineralizing fluids. Commonly white, gray, brown, black, or reddish. Pyritized fossils can be metallic gold or brassy yellow. Silicified fossils may be translucent or opaque white/gray.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable. Can be dull, earthy, waxy, or vitreous (glassy) if silicified. Pyritized specimens will have a metallic luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, ribbed, or spiny, reflecting the original shell ornamentation. The surrounding rock matrix will have a texture characteristic of the sedimentary rock (e.g., gritty for sandstone, smooth for shale, crystalline for limestone).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the fossil itself, as it is an organismal remnant. The shell structure is typically preserved as microcrystalline calcite or replaced by other minerals. Individual mineral crystals may be visible if replacement is extensive.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the fossil as an organism. If the shell is preserved as calcite, it will exhibit rhombohedral cleavage (3 directions at 74° and 106°). If replaced by other minerals, their characteristic cleavage will be present.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Predominantly marine sedimentary rocks, including limestones, shales, sandstones, and siltstones. Found in ancient shallow marine shelf environments, deeper water settings, and reef complexes. Rarely found in brackish or freshwater deposits.

Key Facts

Hardness

Variable, depending on the replacement mineral. Calcite: 3, Silica: 7, Pyrite: 6-6.5. The original shell material (calcium carbonate) is relatively soft.

Specific Gravity

Variable, depending on the replacement mineral and infilling. Calcite: 2.71, Silica: 2.65, Pyrite: 4.95-5.10.

Crystal System

Not applicable to the fossil as an organism. If replaced by minerals, it will exhibit the crystal system of the replacement mineral (e.g., trigonal for calcite, trigonal for quartz).

Color

Highly variable, reflecting original shell and diagenetic alteration.

Luster

Variable, from dull to vitreous to metallic.

Transparency

Opaque to translucent, depending on preservation and replacement minerals.

Fracture

Conchoidal (if silicified), uneven, or splintery, depending on the replacement mineral and preservation.

Cleavage

Not applicable to the fossil itself. If preserved as calcite, it will show rhombohedral cleavage. If replaced by other minerals, their characteristic cleavage will be present.

Composition

Primarily calcium carbonate (CaCO3) in the original shell, often replaced by calcite (CaCO3), silica (SiO2), or pyrite (FeS2) during fossilization. Some early brachiopods had phosphatic shells (calcium phosphate).

Physical characteristics

• Crystal Habit: Not applicable to the fossil. The shell morphology is the primary characteristic.
• Cleavage Type: Not applicable to the fossil. Cleavage refers to the replacement mineral.
• Fracture Type: Variable, depending on the replacement mineral.
• Tenacity: Brittle
• Luster Type: Variable (e.g., earthy, vitreous, metallic)

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