Time Scavengers

Pyrite disease is also called pyrite rot or pyrite decay but is most accurately called pyrite oxidation.

How Do Fossils and Pyrite Interact?

Pyrite, commonly called ‘fool’s gold’,  is the most abundant sulfide mineral (a mineral with sulfide, S^2-, or disulfide, S₂^2-, as the major negatively charged ion (anion) group). Pyrite, or iron disulfide, has the formula FeS₂. Pyrite is found in quartz veins, coal mines, sedimentary rock, metamorphic rock, and in fossils as a replacement mineral. Pyrite forms in oxygen-poor environments, which makes it a good paleoenvironmental indicator. An example of an oxygen-poor (anoxic) environment with high iron and sulfur content today is the deep ocean, especially areas around hydrothermal vents. This means that pyrite is found in association with many fossils that were preserved in deeper ocean waters. 

The original minerals in fossils can be replaced by pyrite. This happens through a process called pyritization, where the original material of the organism (in some cases, even soft parts, like organs) is replaced by pyrite (this process is called replacement diagenesis; diagenesis can be defined as anything that happens to an organism after death as it transitions into a fossil). For instance, due to the anoxic and high-sulfide concentration in the deep ocean, calcite shells can be replaced by pyrite. This replacement process is aided by the presence of sulfur-reducing bacteria (i.e., bacteria that, in the absence of oxygen, convert elemental sulfur from decaying organic material to hydrogen sulfide² [S⁰ + H₂ -> H₂S]). This plays an important role in the creation of pyrite by facilitating the precipitation of sulfides, which react with iron to create iron sulfides during decay¹. In special circumstances, high bacterial activity has even led to documentation of pyrite formation on the shells of living mollusks (the group that includes clams and snails)³. Replacement can lead to beautiful pyritized fossils that look like metal casts or replicas but are actually skeletal material completely replaced by pyrite (Fig. 2). 

Not all fossils end up beautifully pyritized in a way that preserves every detail (Fig. 3), and not all pyritized fossils stay that way. Pyrite disease, also called pyrite oxidation, pyrite rot, or pyrite decay, is a form of rust. Pyrite disease appears in museums and personal collections when specimens that were previously in anoxic conditions are exposed to high oxygen, and often high humidity, conditions for the first time. Millions of years of geologic history have been preserved in the rock record only to be destroyed by poor archiving, humidity, and pyrite decay. 

What Happens When Pyrite Oxidizes?

Since pyrite is formed in anoxic (low oxygen) conditions, most pyritized specimens remain stable for a long time, if they remain in anoxic settings. However, we often see pyrite decay in fossil collections, like museums. The reason we observe so much pyrite decay in museum settings and other fossil collections is because the specimens have been removed from the rock and sediment that were limiting their exposure to oxygen. Pyrite decay happens most often in humid air, where pyrite reacts with both oxygen and water vapor. Although the oxidation does not require water, the reaction with water vapor creates not only iron oxide (‘rust’; Fig. 1, 4), but also iron sulfate, sulfuric acid, and sulfur dioxide gas⁴ (corrosive and toxic materials)¹. This chemical reaction eventually destroys the fossil specimen through a combination of factors. The sulfuric acid is corrosive and damages the specimen, label, and other storage items in contact with the specimen. Oxidized pyrite is unstable and may crumble over time.

Pyrite disease is a continual series of chemical reactions that affects fossils over time. Many pyritized specimens may last for years in perfect condition, while many others progressively decay from pyrite disease that can go undetected for periods of time. The reason that pyrite replacement is notorious for decay has to do with the crystal structure of pyrite. Pyrite often forms a cubic crystal (Fig. 5), which is the stable form of pyrite⁴, since the compact form does not easily absorb moisture. However, pyrite can also occur in other, less stable and more porous forms⁵. Due to several variables, certain geologic formations produce more stable pyrite than others, and not all fossils are equally susceptible to unstable pyrite. There is some research on the differences in decay experienced by stable and unstable forms. Testing may be used to determine what the source of decay is, as different forms of pyrite may appear very similar in cases of replacement. Ammonites, oceanic fossils that often have at least some pyrite replacement, especially those from the Charmouth clays (Dorset, UK), are notorious for pyrite decay while South American specimens and Yorkshire coast (UK) fossils are much more stable².

The iron sulfate that is produced by the oxidation process is considered an efflorescent mineral⁵, a term that means “flowering.” This is because this material has a higher molar volume (the ratio of the volume occupied by a certain amount of a substance) than iron pyrite and wedges the specimen apart as it expands past the constraints of the original crystal structure⁵. Unfortunately, the creation of efflorescent minerals, including other hydrous sulfate minerals, creates a positive feedback loop. The efflorescent materials produce acids and provide water that causes the pyrite to further decay, producing more Fe³⁺ and more efflorescent materials, which feed the cycle of oxidation⁵ (Fig. 6, 7).

What Does Pyrite Disease Look Like?

Pyrite disease may appear gray, greenish, black, red-orange, white, or sulfur yellow. A redder color indicates rust from other iron minerals other than pyrite. Since pyrite contains sulfur and pyrite oxidation produces sulfuric acid, it may have the rotten-egg smell often associated with sulfur. A metallic-iron smell may also be present. The early stages of pyrite disease may appear as spots of discoloration on a fossil, a slightly duller appearance, additional crystallization, or cracking (Fig. 8). Often, the first noticeable sign that is clearly identifiable is dust in the bottom of the box the specimen is in. In these early stages, the severity of the pyrite decay may not be detected until the specimen is moved or handled, and subsequently begins to crumble or leaves dust and a metallic smell on your hands. Later stages appear like mold and may have a fuzzy, cloud-like appearance (Fig. 9). In the worst stages of pyrite decay, what is left is not a fossil that may crumble if handled, but a pile of powder. Pyrite disease can affect vertebrate fossils, invertebrate fossils, botanical specimens, mineral specimens with pyrite, and even the containers and shelving housing the fossils.

It is important to note that inhaling the byproducts of pyrite decay can be hazardous. Pyrite disease is accelerated in humid conditions, and these same conditions may encourage the growth of mold, too. 

Why Is It A “Disease?”

If the damage to the fossils themselves was not bad enough, consider another factor: it’s often called pyrite disease instead of pyrite oxidation for a reason. That reason is that pyrite disease progresses over time, it is irreversible, and it can spread. Historically it was thought that this was the result of a bacterial component, and it was even recommended to treat fossils with antibacterial ointments⁵, but bacterial origin hypotheses are no longer supported. The spread of pyrite disease is due to the spreading of acid and the flaking of iron sulfates that may trigger decay on other specimens that already contain pyrite. To an extent, that makes this a contagious process. The sulfuric acid and sulfur dioxide created in pyrite oxidation can damage the containers that the fossils are in and fossils near the contaminated specimen. 

The expansion of these oxidation products is one of the most damaging aspects of pyrite disease, as the instability in the crystal structure of the specimen causes cracking and flaking. This is because pyrite oxidation causes the conversion of iron sulfide (FeS₂), an iron sulfide mineral that has a cubic crystal structure, to iron sulfate (FeSO₄), which has an orthorhombic crystal structure (Fig. 10). Not only does this cause the specimens to crack and crumble, but the cracking of the specimen causes very small flakes of pyrite to break off and spread. These flakes may land on other specimens, spreading the contamination of pyrite decay to other fossils that were previously stable. In many cases, museums must face difficult decisions to remove scientifically valuable specimens, or even portions of specimens, to prevent further spread of pyrite disease. 

How Can Pyrite Disease Be Managed?

Now, I’ve said that pyrite disease is irreversible. But can it be stopped from progressing once it contaminates a specimen? The simple answer is no. The more complicated answer follows:

Several factors contribute to pyrite disease: temperature, humidity, oxygen levels, pH, exposure to air, crystal structure, and bacteria³. Some factors, such as bacteria and pH, require specific conditions that are uncommon (e.g., the bacteria that can contribute to pyrite disease, Thiobacillus, is only present in environments with more than 95% humidity³ and pH is often affected by the production of sulfuric acid in the decay process). Other factors, those that are more commonly found to contribute to pyrite disease, are closely linked (e.g., temperature and humidity).

Pyrite disease is most closely linked to humidity and high oxygen conditions. The best way to stop pyrite disease is prevention. Most museums use some form of climate control to preserve their specimens, but the issue here is that vertebrate fossils (bones) have a moderate recommended relative humidity level (~45-55%) in order to avoid cracking or warping⁶, while the recommended humidity level to prevent pyrite disease is 30%⁷ (although below 50% is also stated by some sources^2,8).Humidity above 60% is known to accelerate the decay, even to the point that decay can begin within a few days of exposure^{3, 2}. The recommended 30% humidity level to prevent decay is not always possible, especially for public display (like in museums), although it is adhered to in some invertebrate collections. Using things like silica gel packets that are able to reduce moisture and oxygen levels (often called humidity and oxygen scavengers) when in the field collecting (Fig. 11) can reduce the exposure of specimens to these factors. Using sealed containers and other methods that act as barriers to humidity and oxygen when storing specimens can also prevent pyrite disease³. It is important to start the prevention even before the specimen is stored, because the contained storage of an already contaminated specimen may risk trapping moisture and speeding up decay.

Currently, the use of humidity and oxygen buffers is the most common method to prevent or slow pyrite disease- however, there are other methods that are less common. Since the damage from pyrite disease is exacerbated by its byproduct, sulfuric acid, one of the other prevention methods used relies on acid neutralization⁵. There are two common methods to neutralize sulfuric acid. These methods involve the use of ammonium gas or ethanolamine thioglycolate to remove pyrite byproducts (ammonia converts the iron sulfate to iron oxide⁸) and neutralize any generated sulfuric acid². Both methods are intensive, potentially hazardous, and relatively expensive. As a ‘cure’, they are effective at temporarily halting the progression of pyrite disease, but not preventing it or reversing it. These treatments have been proven to be ineffective at preventing pyrite decay without the use of controlled microclimate (oxygen and humidity exposure)³.

Another preventative method is removing salts from the specimen (which may speed up the decomposition) and removing any matrix that may contain pyrite or may trap moisture (e.g., clays)⁵. Washing specimens to remove salts or matrix remains, however, could damage specimens that are already unstable or could cause damaging moisture exposure (although this could be avoided by washing with alcohol). 

Many studies have attempted to find effective ways to stop, reverse, or prevent pyrite decay. From acid treatments to preemptively coating specimens in resin, these tested methods were often damaging to the specimens, especially over the long term. However, were they damaging enough to be worse than pyrite disease? Not necessarily, but a combination of the damage done, the cost, the time investment, and the ineffectiveness of these treatments means that prevention is still the only reliable method for mitigating pyrite disease. 

Coating specimens in resins and varnishes may provide a buffering effect (Fig. 12), but the resin only slows oxidation– it does not stop it. This becomes an issue when the vanishes cannot be removed, and the specimen material cannot undergo other treatment methods². Varnishes may delay the effects, but they have effects of their own, are usually not moisture-proof, and may yellow and crack over time. Coating with resin or embedding the fossil in resin creates the risk of an explosion, too, as the oxidative reaction builds heat and pressure. Some museums coat their fossils in plastic glues (such as Paraloid B-67 or B-72): these glues have the advantage of being reversible, and B-67 repels water, unlike most resins.

Summary:

The destructive products of pyrite oxidation are a concern of museums, miners, and hobbyist fossil collectors alike. Understanding pyrite decay will allow us to better preserve our fossil specimens, and the insights they provide to Earth’s history. Protecting these fossils starts with prevention, and a better understanding of how to manage pyritized specimens.

References:

¹ Replacement/Recrystallization (petrifiedwoodmuseum.org)

² Pyrite Decay in Fossil Collections – ZOIC PalaeoTech Limited (zoicpaleotech.com)

³ Minerals | Free Full-Text | Pyrite Decay of Large Fossils: The Case Study of the Hall of Palms in Padova, Italy (mdpi.com)

⁴Pyrite Disease – Canadian Museum of Nature

⁵Pyrite disease (palaeo-electronica.org)

⁶Shells Eggs Bone and Related Materials 160229 (welshmuseumsfederation.org)

⁷PowerPoint Presentation (vertpaleo.org)

⁸Trouble with pyrite – Deposits Mag