A pyritic ammonite collected on the beach at Charmouth, UK. Photograph kindly shared with us by Martin Curtis of Jurassic Coast Guides.
Pyrite Decay (Pyrite Oxidation)
Treating and Storing Pyritic Fossils in the Amateur Fossil Collection
Pyrite ammonites as found on the beach at Charmouth. You'll notice that they aren't a super bright, glossy gold. This is normal for this type of preservation from this locality. Many of these ammonites can be found just by sitting down on the beach and sifting through the pebbles. The tide sorts fossils by density, and so you'll often see little patches of pyritic lumps. Some of these will be ammonites! Photo Credit: Martin Curtis, Jurassic Coast Guides.
A pyritic ammonite, hiding away in a larger lump of amorphous pyrite. Photograph from John Burrows of Albion Fire and Ice.
Dactylioceras ammonite found on the Holderness Coast. This split was achieved using just a hammer, likely down to the high pyrite content making a plane of weakness between fossil and rock. Photo from Scott Taylor.
A series of ammonites collected from Charmouth beach. These were varnished by the owner to seal them from the elements (left), they started to go (middle) with the varnish lifting off, and then before long he was left with just a few piles of dust (right). Looks like he needs to go out collecting again! Photo Credit: Chris Andrew
These Creniceras renggeri ammonites were collected over 100 years ago. See the white and yellow powder? The label has been attacked by the sulphuric acid released in the process of pyrite decay. This means that any important information stored with the fossil could also be lost. Photo credit: David Ware.
This is the back of a nodule where the whole nodule is decaying due to its high unstable pyrite content. This rock has an acrid smell due the sulphuric acid released during the reaction. Photo Credit: Chris Andrew
This cut and polished ammonite would have been beautiful when purchased, with the pyrite showing up as golden glinting areas. Sadly, it too has succumbed. Most reputable dealers won't sell fossils that are known to decay quickly, but there are always some that slip through the net unbeknownst to the seller. Photo Credit: Angie Lester
So what is Pyrite Decay?
Pyrite disease or pyrite decay can completely destroy specimens. In the presence of oxygen, pyrite (FeS2) breaks down to ferrous sulphate (FeSO4) and sulphur dioxide (SO2). Where water or atmospheric moisture is present, sulphuric acid (H2SO4) is also produced as a biproduct. If you're interested in the chemistry, a quick Google search of 'Pyrite Oxidation' will give you the in-depth process.
Marcasite is a dimorph of pyrite found in sedimentary rocks, which is less common but is also unstable. Moisture levels as low as 60% and upwards can trigger an accelerated rate of decay. Unfortunately for us, this is not an abnormal British summer day. This yellow/white dust, the hydrated sulphates and sulphuric acid are potentially a health hazard, as well as being destructive to your fossils.
But I have lots of pyritic fossils, and they're all fine? Pyrite can occur in two forms – compact, well crystallised and stable or porous, microcrystalline and impure and very unstable. The latter of the two can decay extremely quickly. The reason it is largely stable when in the rock is because contact with oxygen is extremely if not totally limited.
Photograph kindly shared with us by David Ware. These ammonites are from the Creniceras renggeri marls of the Jura Mountains, visually separated into 4 stages of decay. You can even see that the label has been damaged by the sulphuric acid released.
Pyrite in its cubic form mixed with quartz. These cubes are typically very stable. Photograph taken in the National Museum of Geology in Bucharest.
Pyrite in its less stable form. You can see that this example is much more porous looking, and that it is decaying. You can see the dust surrounding the specimen in the cabinet. Photograph taken in the National Museum of Geology in Bucharest.
This is one reason why fossils from the Yorkshire and Holderness Coast, whilst heavily pyritic, can and have lasted centuries in fossil collections without treatment, but others are much more susceptible to fast decay. However, there are still many specimens that start to 'bloom' with slower deterioration.
The pyrite cubes from South America seem almost impervious to decay. Pyritic Echioceras ammonites from Charmouth are notorious for it. This is nothing to do with the species of ammonite, it’s just the crystal structure of the pyrite in that particular layer of clay. The ammonites from other layers (Eoderoceras, Oxynoticeras etc) are far more stable, although even they can decay.
Echioceras ammonite from Charmouth, notorious for decaying rapidly due to the crystalline structure of the pyrite. Photo: Martin Curtis.
Pyrite oxidation products can expand within and crumble or crack rocks and fossils. This can happen within individual specimens, or even within pyrite rich rocks. The birchi beds at Lyme Regis have a tendency to do this. The darker and more pyrite rich the matrix, the more this is a problem. It is said that the birchi beds at Charmouth are more stable than those on Monmouth beach to the west of Lyme Regis. The outer layers of the nodule seem most susceptible to cracking so removing excess matrix is often useful. Also the thicker the rock the more stress builds up due to any expansion or contraction of the matrix. Another lithology prone to cracking is “goldstone”, a pyrite rich layer full of specimens of Arnioceras. Remove whatever rock you can to prevent this.
Sometimes difficult decisions must be made when it comes to removing rock or even parts of a fossil. Below is an ichthyosaur fossil from Lyme Regis, comprising much of the skeleton (collected over many years by different people), with skin and soft tissue preservation. Whilst not entirely uncommon for the locality, this is undeniably a very special fossil. Unfortunately, the skin and soft tissue is particularly pyritic and has rapidly started to decay, and so most people would say to remove the unstable material to save the rest (the bones). There is no technique that can be used that doesn't destroy the skin and soft tissue to separate it, and as it is it is in 'self destruct mode'. These are challenges faced by amateur collectors, and the largest museums. In some cases, a photographic record is all that is left.
The sulphuric acid and spread of it (it eats through paper, card, wood and lifts paint) is why it is sometimes referred to as a disease - it looks like it is creeping and spreading. Unlike a disease, it doesn't actually spread. It is more likely that similarly susceptible specimens (same locality, same composition, or parts of the same overall specimen) are being stored in the same environment. Indeed, it was for this reason that many people in the past believed that it was a bacterial problem, which is now known not to be true. However, you may still be told to apply antibacterial cream to your fossils - don't. It won't achieve anything other than depleting your first aid kit.
How can I prevent pyrite decay from happening?
There are two known CURES to pyrite oxidation.
Pyrite Stop and Ammonium Gas treatment
There are two known 'cures' to the problem. Whilst they cannot reverse damage already done, they can remove the biproducts of decay and neutralise the specimen from the sulphuric acid generated. In essence, they can halt it in its tracks. If the fossils are then stored in an appropriate environment, no further decay should occur. It is always best to tackle any potential problem before it begins, but that is not always an option.
These two methods are treatment with ammonium gas, or with our Pyrite Stop (ethanolamine thioglycolate). In the home workshop, treatment with Pyrite Stop is preferable as ammonium gas (although effective) is both toxic and flammable, and any inhalation even in low volumes can cause extreme respiratory distress.
Pyrite preserved bone material from the Garden Cliff (Westbury-on-Severn) collected and conserved by Tom Carr. You can see the high pyrite content, with masses of pyritic cubes. This preservation is notoriously unstable. Tom treated the specimen with ammonium gas, and it is two years strong and still going. Ammonium gas is used to neutralise the sulphuric acid, with specimens suspended above a solution of ammonium hydroxide in Polyethelene Glycol in an enclosed environment. Photo: Tom Carr
Using Pyrite Stop - A Guide
Cornish & Doyle (1984) experimented with the use of ethanolamine thioglycolate to prevent this oxidation and therefore pyrite decay. Although a well known treatment for pyrite decay, this substance has been near impossible for the average preparator to get hold of in Britain for many many years, so we are finally able to bring you the solution to your pyrite problems. So, why Pyrite Stop? How does it work? Pyrite Stop is a pyrite stabiliser.
It is alkaline and therefore neutralises and washes out sulphuric acid (a product of pyrite decay)
Thioglycolates react with soluble and insoluble iron compounds, chelates and complex iron, but not stable pyrite. These are the oxidation by-products. This reaction generates a purpley solution of ferrothioglycollate.
It is soluble in ethanol or anhydrous isopropanol – as are the products of pyrite decay. Thus, contact with water (which may be damaging) can be avoided.
Pyrite Stop - Immersion
8. Anything that needs reassembly can be glued back together. Specimens can also be coated once completely dry to provide a protective barrier. It is well known that no substance we can use on fossils is impermeable to moisture and oxygen, but Paraloid B-72 can delay the process, as well as being completely removable if and when retreatment in Pyrite Stop is required. Thin coats are better than one thick coat.
9. Once treatment is completed, you will still need to store your specimens in a very low humidity environment, ideally with dessicants (like silica gel) and oxygen scavengers or absorbers. A sealed cabinet or container where you can keep humidity below 45% is ideal. You may need to retreat them in the future, but these conditions are the most conducive to keeping your pyritic fossils stable.
A Pyrite Stop Paste - where immersion is not possible
For large specimens, or friable specimens an alternative technique can be used. It takes time, and is often not as successful as immersion. A solution of 3-5% ethanolamine thioglycolate is mixed with sepiolite (magnesium silicate – which in its impure form is used as cat litter) to form a paste. Apply liberally and wrap in foil or plastic sheeting to prevent evaporation of the isopropanol. After 2-3 hours, allow the paste to air dry. The sepiolite is then removed by gently brushing. Apply a paste of pure isopropanol and sepiolite to clean the specimen or thoroughly wash the area with alcohol. Gauze can be used to separate the sepiolite and the surface of the specimen so removing the dried sepiolite is easier. The trouble with this technique, especially with larger specimens, is that penetration into the specimen is limited.
What NOT to use. A minefield of misinformation.
Not a 'disease' and not caused by bacteria
Do not apply nail varnish or any other varnish
- Do not apply any other kind of non-reversible varnish!
Don't soak or store specimens in water
Store in a dry environment
Preferably store with oxygen scavengers and dessicants
Use Pyrite Stop
- Clean the fossil after finding it
DO NOT USE substances such as nail varnish or yacht varnish. These will be detrimental to the specimen and almost certainly impede conservation efforts in the future as they are not fully reversible and contain many impurities. In many cases, they are not moisture proof. They might delay the reaction for the time, but these efforts will likely not stop the problem and will eventually create future problems. Many varnishes yellow and crack over time (especially nail varnish which is designed for short term wear). Once this is combined with pyrite decay, you are unlikely to be able to recover the fossil. Our non-yellowing Fossil Varnish is brilliant for finishing calcite fossils, but we wouldn't recommend it for pyritic fossils.
If you are going to coat your fossil, use Paraloid B-72 or a similarly reversible substance. That way you can remove it for a round of treatment in Pyrite Stop, and then reapply once it is finished. Paraloid B-72 will not yellow or crack, and is used in virtually every museum and institution worldwide.
These ammonites have had varnish applied to 'seal' them from the atmosphere (oxygen and moisture). The varnish is the yellowed, peeling stuff that you can see. In the middle of the ammonite on the left, it is not that yellowed, but still visibly slightly bulging. The idea is that varnish is not a suitable barrier between the ammonite and the atmosphere, and is virtually impossible to remove until the ammonite is so far gone that it cannot be salvaged; and in some cases with a number of additives may actually exascerbate the speed of pyrite decay. The addition of nitrocellulose to many nail varnishes will cause yellowing and cracking. The cracking will allow in the air and moisture, and is not easy to remove particularly on a fragile surface. Using a substance like Paraloid B72 at least allows you to remove it before treatment in Pyrite Stop. Photos: Chris Andrew.
At one point it was though that it was caused by a bacteria called Acidithiobacillus (previously Thiobaciullus), and so substances like arsenic and Savlon (an antibacterial cream typically applied to wounds) have been used to try and prevent it. Bacteria, where present, might catalyse pyrite decay, but this is highly unlikely in most circumstances to be the cause unless the relative humidity is above 95%. Therefore, using an antibacterial cream is no more than an old wives tale at this stage. This advice is still fairly prevalent online.
Embedding in Resin
Best avoided. A lot of collectors have come up with this idea independently of one another. It's a sure fire way to prevent reaction with the atmosphere and moisture? Right? Many collectors before you who have tried this will tell you that in the end it will probably just explode under pressure. The crystalline decay products are several times the volume of the original specimen, and the likelihood of you being able to eliminate all oxygen and moisture from within the fossil before embedding it in resin are next to nil (and the oxidation process itself generates water, as well as oxidised iron, which makes the process self-propogating). If it doesn't explode, you're stuck with some hideous lump of gnarled resin with a bubbly thing in the middle, which you then can't remove the resin to get at and treat. Most resins are not removable, and many are known for yellowing over time (e.g. epoxy resins).
Further reading (click the links below to read the articles):
Larkin, N. (2011). Pyrite Decay: cause and effect, prevention and cure. NatSCA News, Issue 21, 35 ‐ 43. This is an excellent easy-reading review of what we know now about Pyrite Oxidation from an extensively knowledgeable source in the field of conservation, Nigel Larkin of Natural History Conservation.
Cornish, L. & Doyle, A. (1984). Use of ethanolamine thioglycollate in the conservation of pyritized fossils. Palaeontology, Vol. 27, part 2, 421-424.
The photographs used in this article that are not our own have been reproduced with the permission of the photographer. Please do not use, print or reproduce these photographs without the express permission of the photographer, they are under copyright.