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Ammonite fossils in shale consist of the preserved remains of ammonoids embedded within a fine-grained, laminated sedimentary rock. The ammonite shells are typically coiled, often exhibiting intricate suture patterns (the lines where internal septa meet the outer shell wall). The shale matrix is generally dark gray to black, reflecting its organic content, and has a fissile (splits into thin layers) nature. The preservation quality of the ammonite can vary from flattened impressions to three-dimensional, mineralized shells.
How to Identify
- Color
- Shale matrix is typically dark gray, black, brown, or greenish-gray. The ammonite fossil itself can be white, gray, brown, black, golden (pyritized), or iridescent (aragonite preservation).
- Luster
- Shale matrix is dull to earthy. Ammonite fossil can be dull, earthy, vitreous (if calcified or silicified), or metallic (if pyritized), or pearly/iridescent (if original aragonite is preserved).
- Texture
- Shale is fine-grained, smooth to slightly gritty, and often fissile (splits into thin layers). The ammonite fossil will have a distinct, often ribbed or ornamented, coiled shell texture.
- Crystal Form
- Shale is an aggregate of microscopic clay minerals and other detrital grains, lacking macroscopic crystal form. The ammonite fossil exhibits the characteristic planispiral or heteromorphic coiling of the cephalopod shell.
- Cleavage
- Shale exhibits excellent parting along bedding planes (fissility), but individual mineral grains lack macroscopic cleavage. The ammonite fossil itself does not exhibit cleavage in the mineralogical sense, but may fracture conchoidally if silicified or calcified.
- Geological Environment
- Marine sedimentary environments, particularly low-energy, often anoxic, deep-water or shelf settings where fine muds accumulate and organic matter is preserved. Common in ancient epicontinental seas and ocean basins.
Key Facts
- Hardness: Shale: 2.5-4 (Mohs). Ammonite fossil: Varies depending on preservation mineral (e.g., Calcite 3, Pyrite 6-6.5, Silica 7).
- Specific Gravity: Shale: 2.0-2.8. Ammonite fossil: Varies depending on preservation mineral (e.g., Calcite 2.71, Pyrite 4.95-5.10, Silica 2.65).
- Crystal System: Shale: Not applicable (aggregate of microscopic minerals). Ammonite fossil: Not applicable (biogenic structure, though replacement minerals have their own crystal systems).
- Color: Shale: Dark gray, black, brown. Ammonite: White, gray, brown, black, golden, iridescent.
- Luster: Shale: Dull, earthy. Ammonite: Dull, earthy, vitreous, metallic, pearly.
- Transparency: Shale: Opaque. Ammonite: Opaque.
- Fracture: Shale: Splintery, conchoidal (rare). Ammonite: Conchoidal (if silicified/calcified), irregular.
- Cleavage: Shale: Excellent parting (fissility). Ammonite: None (as a fossil structure).
- Composition: Shale: Predominantly clay minerals (kaolinite, illite, smectite), quartz, feldspar, organic matter, and other detrital grains. Ammonite fossil: Original shell was aragonite (CaCO3), often replaced by calcite (CaCO3), pyrite (FeS2), or silica (SiO2).
Quick Check
- Color: Dark gray to black shale with a coiled fossil of varying color (white, brown, golden, iridescent).
- Luster: Dull to earthy shale, fossil can be dull, vitreous, metallic, or pearly.
- Streak: Shale: White, gray, or brown. Fossil: Varies depending on mineral composition (e.g., white for calcite, black for pyrite).
Physical Characteristics
- Crystal Habit: Shale: Microcrystalline to cryptocrystalline aggregate. Ammonite: Biogenic, coiled shell morphology.
- Cleavage Type: Shale: Excellent parting along bedding planes (fissility). Ammonite: None.
- Fracture Type: Shale: Splintery, irregular. Ammonite: Conchoidal (if mineralized), irregular.
- Tenacity: Shale: Brittle. Ammonite: Brittle.
- Luster Type: Shale: Dull, earthy. Ammonite: Variable (dull, vitreous, metallic, pearly).
Formation
Ammonite fossils in shale form when the remains of ammonoids, an extinct group of marine cephalopods, are rapidly buried in fine-grained marine sediments (mud). These sediments, rich in organic matter and clay minerals, accumulate in low-energy, often anoxic, marine environments such as deep-sea basins, continental shelves, or epicontinental seas. Over geological time, compaction and diagenesis transform the mud into shale, preserving the ammonite shells as fossils. The shells themselves are typically replaced by minerals like calcite, pyrite, or silica, or preserved as molds and casts.
Usage
Primarily scientific and educational (paleontology, stratigraphy, evolutionary studies). Also collected by hobbyists and used as decorative items or in jewelry, particularly well-preserved or iridescent specimens.
Age Distribution
Lower Devonian to Upper Cretaceous (approximately 400 to 66 million years ago). Most abundant in Jurassic and Cretaceous periods.
Where to Find
Jurassic Coast, Dorset and East Devon, UK
World Heritage Site famous for abundant and well-preserved ammonites in shales and limestones, particularly from the Lower Jurassic (Lias Group).
Bearpaw Shale, Western Interior Seaway, North America
Extensive Cretaceous marine shale formation across the Great Plains of the USA and Canada, yielding numerous large and often iridescent ammonites (e.g., Placenticeras, Baculites).
Posidonia Shale, Germany
Lower Jurassic (Toarcian) black shale renowned for exceptional preservation of marine fossils, including ammonites, with soft tissues occasionally preserved.
Madagascar
Known for beautifully preserved, often iridescent ammonites (e.g., Cleoniceras, Douvilleiceras) from Cretaceous deposits, frequently found in a shaly or marly matrix.
Kimmeridge Clay, UK
Upper Jurassic black shale formation, particularly in southern England, rich in ammonites and other marine fossils.
Finding Tips
Target Marine Sedimentary Rocks
Focus on outcrops of fine-grained marine sedimentary rocks, especially shales, mudstones, and marls, that are known to be of Devonian to Cretaceous age.
Look for Distinctive Shapes
Search for coiled, spiral, or sometimes straight (heteromorph) shapes that stand out from the surrounding rock matrix. Ribbing or ornamentation on the shell surface can also be a key indicator.
Examine Weathered Surfaces
Weathering can sometimes highlight fossils by eroding the softer matrix or by differential staining. Look for subtle relief or color changes.
Check for Pyrite or Calcite Infillings
Ammonites are often preserved as internal molds or replaced by minerals like pyrite (golden metallic luster) or calcite (white to clear crystalline). These can be more visible than the original shell material.
Safety Precautions
Always obtain permission before collecting. Be aware of unstable cliffs and slippery surfaces. Wear appropriate safety gear, including eye protection and sturdy footwear. Some shales can contain pyrite, which can oxidize to sulfuric acid, so wash hands after handling. Avoid inhaling dust if breaking rocks, as shale contains silica.
Similar Rocks
Ammonite in Limestone
Ammonoidea (fossil) within Limestone (rock)
Also known as: Ammonoidea in Limestone
Nautiloid in Shale
Nautiloidea (fossil) within Shale (rock)
Also known as: Nautilus in Shale
Bivalve in Shale
Bivalvia (fossil) within Shale (rock)
Also known as: Clam in Shale
Scientific Classification
- Mineral Class
- Not a single mineral; a rock (shale) containing a fossil (ammonite).
- Group
- Sedimentary Rock (Shale) and Cephalopod (Ammonoidea)
- Crystal System
- Not applicable (rock and fossil).
- Chemical Formula
- Shale: Variable (complex silicates, oxides, carbonates). Ammonite: Original shell CaCO3 (aragonite), often replaced by CaCO3 (calcite), FeS2 (pyrite), or SiO2 (silica).
- Composition
- Shale: Clay minerals, quartz, feldspar, organic matter. Ammonite: Calcium carbonate, iron sulfide, or silicon dioxide.
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