How to identify Fossilized Shark Tooth in Matrix

To correctly identify Fossilized Shark Tooth in Matrix (Chondrichthyes tooth 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

   Tooth: Typically black, dark brown, gray, or reddish-brown due to mineral replacement. Matrix: Highly variable, reflecting the composition of the host rock (e.g., gray, brown, tan, white, red, green).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Tooth: Sub-vitreous to dull, sometimes slightly waxy. Matrix: Dull to earthy, depending on lithology.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Tooth: Smooth to slightly granular, often with preserved serrations or enameloid texture. Matrix: Variable, from smooth (shale) to gritty (sandstone) or rough (conglomerate).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Tooth: Biogenic, not a true crystal form, but retains the original tooth morphology (triangular, lanceolate, cuspidate, etc.). Matrix: Crystalline (e.g., quartz grains) or amorphous (e.g., clay minerals) depending on the rock type.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Tooth: None (fractures conchoidally or irregularly). Matrix: Variable, depending on the rock type (e.g., shales exhibit fissility, sandstones have no cleavage).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Marine sedimentary environments, particularly shallow to deep marine shelf deposits, estuarine environments, and phosphatic lag deposits. Common in formations rich in marine fossils, such as marls, limestones, sandstones, and phosphatic beds.

Key Facts

Hardness

Tooth (bioapatite): Approximately 5 on Mohs scale. Matrix: Variable, depending on the rock type (e.g., quartz in sandstone is 7, calcite in limestone is 3).

Specific Gravity

Tooth (permineralized bioapatite): 2.9-3.2. Matrix: Variable, depending on the rock type (e.g., sandstone ~2.65, shale ~2.0-2.8).

Crystal System

Tooth: Amorphous (biogenic origin, though composed of cryptocrystalline hydroxylapatite). Matrix: Variable, depending on constituent minerals.

Color

Tooth: Black, dark brown, gray, reddish-brown. Matrix: Highly variable.

Luster

Tooth: Sub-vitreous to dull. Matrix: Dull to earthy.

Transparency

Tooth: Opaque. Matrix: Opaque.

Fracture

Tooth: Conchoidal to irregular. Matrix: Variable (e.g., splintery in shale, granular in sandstone).

Cleavage

Tooth: None. Matrix: Variable (e.g., perfect in some clay minerals, none in quartz).

Composition

Tooth: Primarily permineralized bioapatite (calcium phosphate, Ca5(PO4)3(OH,F,Cl)), often with secondary mineralization by iron oxides, manganese oxides, or other minerals. Matrix: Composed of detrital grains (e.g., quartz, feldspar), clay minerals, carbonate minerals (calcite, dolomite), or phosphatic material, cemented together.

Physical characteristics

• Crystal Habit: Tooth: Biogenic, retaining original tooth morphology. Matrix: Granular, clastic, platy, or massive, depending on rock type.
• Cleavage Type: Tooth: Not applicable. Matrix: Variable (e.g., basal in micas, none in quartz).
• Fracture Type: Tooth: Conchoidal to irregular. Matrix: Variable (e.g., splintery, granular, hackly).
• Tenacity: Tooth: Brittle. Matrix: Variable (e.g., brittle in sandstone, friable in some shales).
• Luster Type: Tooth: Sub-vitreous to dull. Matrix: Dull to earthy.

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