What Are Pyrite Ammonites

Pyrite ammonites are fossilized shells of extinct marine mollusks whose original material has been replaced or coated by iron sulfide (FeS₂), more commonly known as pyrite or “fool’s gold.” This rare type of fossilization, called pyritization, gives the ammonite a striking metallic sheen ranging from pale brass to deep gold. While ordinary ammonite fossils are common worldwide, pyrite ammonites are exceptional both for their rarity and their exquisite preservation.

What Are Ammonites?

Ammonites were marine cephalopods—relatives of modern squids, cuttlefish, and nautiluses—that lived from the Devonian Period (~419 million years ago) to their extinction at the end of the Cretaceous (~66 million years ago). They thrived in oceans worldwide, using their coiled, chambered shells for buoyancy control and protection.

Rare pyritized ammonite fossils with FeS₂ replacement, showcasing metallic gold sheen and detailed shell sutures from mineralization.

How Pyrite Ammonites Form

Pyrite ammonites formed through a rare fossilization process, known as pyritization, that required very specific conditions shortly after the animal’s death. The sequence typically unfolded as follows:

Rapid Burial – After death, the ammonite shell sinks to the seafloor and is quickly buried by fine-grained, organic-rich sediments. This shields it from scavengers, strong currents, and aerobic decay, creating the conditions necessary for preservation.

Low Oxygen Anoxic Environment – The burial site must have little or no oxygen, often in stagnant basins or deeper waters. This slows decomposition by oxygen-dependent organisms and creates a chemically reducing environment where pyrite can form.

Sulfate-Reducing Bacteria – In the absence of oxygen, specialized bacteria use sulfate (SO₄²⁻) from seawater as an energy source, breaking down organic matter from the shell or surrounding sediment. This releases hydrogen sulfide gas (H₂S), the essential chemical precursor to pyrite.

Pyrite Precipitation – Hydrogen sulfide diffuses into the surrounding pore waters and reacts with dissolved iron (Fe²⁺) from iron-bearing minerals or iron-rich fluids. Initially, unstable iron sulfides like mackinawite or greigite may form, but over time, they transform into stable pyrite crystals.

Mineral Replacement and Preservation – Pyrite replaces the original shell minerals (aragonite or calcite) molecule by molecule or infills its chambers and voids before compaction. This early diagenetic mineralization preserves fine structural details and gives the fossil its distinctive metallic golden luster.

This type of preservation is most often found in Jurassic-age marine deposits (about 201–145 million years old), where iron-rich, sulfate-rich muds and low-oxygen waters created ideal conditions for fossilization.

Small pyritized ammonite fossils in limestone, showcasing golden iron sulfide (FeS₂) preservation—Stonebarrow, Charmouth, UK. Photo by: Jason Warren.

Geological Conditions Required

Pyrite ammonites are found only where these conditions existed:

Anoxic, sulfate-rich marine muds – Often in stagnant basins or restricted shelf environments.
High organic content – Fuel for bacterial sulfate reduction.
Abundant reactive iron – From detrital minerals, volcanic ash, or hydrothermal sources.
Stable burial – Protection from oxidation before lithification.

Matched pair of pyritized ammonite fossils.

How They Differ from Other Ammonite Fossils

Most ammonites are preserved in calcite, aragonite, or silica, or appear as simple molds in sedimentary rock. Pyrite ammonites stand out due to:

Metallic Luster – A natural brassy-gold sheen unique to pyritization.
Density – Pyrite is heavier than calcite or silica, giving the fossils a substantial weight.
Detail Fidelity – Pyrite often preserves ribs, sutures, and growth lines in sharper detail than other mineral replacements.
Rarity – Pyritization occurs only in restricted anoxic environments, making these fossils far less common.
Chemical Vulnerability – Pyrite can oxidize (“pyrite disease”), unlike more stable calcite fossils.

Pyrite ammonites (FeS₂) with metallic gold sheen, contrasting calcite or aragonite-preserved fossils.

Notable Localities

Pyritized ammonites, or "golden ammonites," form in anoxic marine sediments where pyrite replaces or coats shells, preserving fine details. These Jurassic–Cretaceous fossils offer insights into ancient oceans. Below are key global sites.

Dorset, UK (Charmouth, Lyme Regis, Monmouth Beach): Lower Jurassic Blue Lias/Charmouth Mudstone. Golden pyritized Promicroceras, Harpoceras, Asteroceras; nodular beach finds.

Whitby, UK: Lower Jurassic Whitby Mudstone. Detailed Dactylioceras, Hildoceras in anoxic shales.

Christian Malford, UK: Middle Jurassic Oxford Clay. Pyrite-encrusted Kosmoceras with soft tissues.

Holzmaden, Germany: Lower Jurassic Posidonia Shale. Large Harpoceras in lagerstätte.

La Voulte-sur-Rhône, France: Middle Jurassic marls. Golden fossils; ammonites often phosphatized with pyrite.

Russia Volga River (Saratov, Ul’yanovsk): Middle Jurassic–Lower Cretaceous shales. Lustrous Quenstedticeras; large pyrite/marcasite specimens.

Morocco: Atlas/Sahara Regions: Upper Cretaceous. Polished pyritized Mammites, Prionoceras in nodules.

Alberta, Canada (Bearpaw Formation): Upper Cretaceous shales. Rare pyrite-coated Placenticeras; mainly iridescent.

Multiple pyritized ammonite species & sizes in iron sulfide (FeS₂)—micro to macro fossils from Stonebarrow, Dorset’s Jurassic Coast.

Why They Matter to Science

Pyrite ammonites are more than striking fossils—they are scientific records of ancient oceans, climates, and life.

Indicators of Past Environments – Their formation in anoxic, iron-rich seabeds reveals past “dead zones” and helps map ancient ocean chemistry.
Exceptional Preservation – Pyritization captures microscopic shell details and internal structures, aiding taxonomy and studies of ammonite anatomy and evolution.
Geochemical Records – Sulfur and iron isotopes in pyrite trace microbial activity, seawater composition, and past climate conditions.
Climate and Event Markers – Layers rich in pyrite ammonites can signal warming events, sea-level changes, or mass extinction triggers.

Pyritized ammonite fossil with intricate suture patterns.

Caring for Pyrite Ammonites

Pyrite ammonites are visually stunning but chemically unstable when exposed to moisture. Over time, humidity can trigger pyrite disease—oxidation that causes cracking, crumbling, and even sulfuric acid formation. Proper care is essential to preserve their beauty and scientific value:

Maintain Low Humidity – Keep specimens in an environment below 40% relative humidity.
Use Protective Storage – Store in sealed cases with silica gel or other moisture absorbers.
Handle with Care – Avoid touching with bare hands to prevent oils and moisture transfer.
Consider Protective Coatings – Apply archival-grade consolidants (such as Paraloid B-72) when appropriate for long-term preservation.

With the right storage and handling, pyrite ammonites can remain stable and striking for generations.

Stunning pyritized Quenstedticeras ammonite - glittering pyrite chambers from Russia's Volga River, Callovian Stage (161 mya). Photo by: Arkenstone/iRocks.com.

Quick Identification Guide

Color – Metallic brassy-gold to silvery-gold.
Luster – Bright metallic, sometimes with cubic crystal faces.
Shape – Spiral, chambered shell typical of ammonites.
Hardness – Mohs 6–6.5 (harder than gold).
Weight – Dense compared to non-pyritized fossils.
Age – Usually Jurassic or Cretaceous

Pyritized ammonite fossils from Charmouth's Jurassic Coast, Dorset.

Summary

Pyrite ammonites are a rare intersection of paleontology and mineralogy—the remains of ancient marine predators transformed into glittering metallic fossils by precise chemical conditions on the seafloor. Found mainly in select Jurassic and Cretaceous formations, they combine scientific significance, aesthetic appeal, and geological rarity. Their golden sheen makes them prized collector’s items, while their preservation provides invaluable windows into the chemistry and ecology of Earth’s ancient oceans.