How to identify Ammonite Fossil

To correctly identify Ammonite Fossil (Ammonoidea fossil in sedimentary rock), 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

   The fossil itself can range from white, gray, brown, black, or reddish-brown, depending on the mineral composition of the fossil and the surrounding matrix. Iridescent colors (ammolite) can be present if the original nacreous layer is preserved or replaced by specific minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Varies from dull (if preserved as a mold or cast in fine-grained sediment) to vitreous, pearly, or metallic (if replaced by calcite, aragonite, or pyrite, respectively).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The fossil surface can be smooth, ribbed, tuberculate, or keeled, reflecting the original shell ornamentation. The surrounding rock matrix will have the texture characteristic of its lithology (e.g., gritty for sandstone, smooth for shale, crystalline for limestone).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   The fossil retains the original planispiral (coiled in a single plane) or rarely helically coiled shell morphology of the ammonite. The internal chambers (camerae) separated by septa, and the complex suture lines, are key diagnostic features. The mineral infill may show crystalline forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the fossilized organism itself, but the minerals replacing the shell (e.g., calcite) may exhibit rhombohedral cleavage. The surrounding sedimentary rock may exhibit cleavage (e.g., slaty cleavage in shale) or parting along bedding planes.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine sedimentary environments, particularly those with relatively calm waters and moderate sedimentation rates, such as shallow to deep shelf environments, epicontinental seas, and basinal settings. Often found in shales, limestones, marls, and sandstones.

Key Facts

Hardness

Varies significantly depending on the mineral composition of the fossil and the surrounding rock. Calcite (3 Mohs), Pyrite (6-6.5 Mohs), Quartz (7 Mohs). The rock matrix will have its own hardness.

Specific Gravity

Highly variable, depending on the infilling mineral and the density of the surrounding rock. Typically ranges from 2.6 to 3.5 g/cm³.

Crystal System

Not applicable to the organism itself. The replacing minerals will have their own crystal systems (e.g., trigonal for calcite, isometric for pyrite, hexagonal for quartz).

Color

Highly variable, from white, gray, brown, black, to reddish-brown, or iridescent (ammolite).

Luster

Dull, earthy, vitreous, pearly, or metallic.

Transparency

Opaque.

Fracture

Not applicable to the fossil itself. The surrounding rock may exhibit conchoidal, uneven, or splintery fracture.

Cleavage

Not applicable to the fossil itself. Infilling minerals like calcite may show rhombohedral cleavage.

Composition

Primarily calcium carbonate (CaCO3) if preserved as calcite or aragonite, or silicon dioxide (SiO2) if silicified, or iron sulfide (FeS2) if pyritized. The surrounding rock is composed of various detrital or chemical sediments.

Physical characteristics

• Crystal Habit: The fossil retains the original planispiral or helically coiled shell morphology. Internal structures like septa and suture lines are often preserved. The infilling minerals may show granular or crystalline habits.
• Cleavage Type: Not applicable to the fossil. Infilling calcite may show perfect rhombohedral cleavage.
• Fracture Type: Not applicable to the fossil. The surrounding rock may exhibit various fracture types.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, vitreous, pearly, or metallic.

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