Pyrite Decay: cause and effect, prevention and cure

Pyrite (FeS2) is a common mineral found in igneous, sedimentary and metamorphic rocks; it may be present in petrology, mineral and palaeontological collections. Pyrite decay, or pyrite oxidation, has been recorded since the 19 century and various methods have been devised over the years to prevent or ’cure’ it with varying degrees of success. Methods of identifying pyrite decay in collections are discussed along with potential problems this can cause to the specimens and associated labels. Up to date prevention methods are discussed, including microclimates, controlled environments, collections surveys and resin coatings. Modern techniques of ‘curing’ pyrite are discussed in detail, including ammonium gas treatment and Ethanolamine Thioglycollate treatment. Introduction Exactly ten years ago the Natural Sciences Conservation Group provided a day-long seminar about pyrite decay at the Natural History Museum in London, consisting of talks and demonstrations. It might be useful to repeat this event or something similar every five or ten years as there will always be some people new to the profession to whom the issue is a mystery, and some who would like a refresher particularly as the subject has a long history of confusion over the exact nature of the processes involved. An example of a late 19 century response to pyrite decay describes how “the preparators mistakenly believed that the Pyrite disease was caused by an unknown germ and developed a method to counteract its effects... The bones were soaked in a mixture of alcohol, arsenic and shellac.... The alcohol was able to penetrate the bones, carrying the arsenic (supposed to kill the mysterious “germ” causing the problem), and the shellac successfully hardened the weaker parts” (Spalding, 1993). Shinya and Bergwall (2007) recently provided a concise summary of what pyrite is, how it decays and (briefly) how specimens may be treated, but their PDF poster included only some of the practical ways of preventing or at least reducing the likelihood of pyrite decay. Some other important preventive measures are described below, along with relevant notes about the nature of pyrite and how it decays. What is pyrite? Pyrite (FeS2), also known as ‘fools gold’, is a common mineral found in sedimentary, metamorphic, and igneous rocks. It grows in crystalline forms, typically cubic or octahedral. The crystal can be several centimetres in diameter for well-grown cubes, or microcrystalline (Howie, 1992). It can occur either compact, well crystallized and stable or porous, microcrystalline, often impure and very unstable. Marcasite (FeS2) is a dimorph of pyrite that primarily occurs in sedimentary rocks. It is not as commonly found as pyrite but it too can be unstable and susceptible to oxidation (Rixon, 1976; Cornish, 1987; Shinya & Bergwall, 2007). Mineral and palaeontological specimens that consist almost wholly of pyrite are easily identified as such, as they look and feel ‘metallic’. It is much less easy to identify those specimens that contain a relatively small amount of pyrite (where it is finely disseminated through the specimen), or in which the pyrite has developed only within the inner pore spaces of what was once bone, for example. How does pyrite ‘decay’? Pyrite oxidation (also known as pyrite ‘disease’, pyrite ‘rot’ and pyrite ‘decay’) is caused and accelerated by the presence of oxygen and water, even in relative humidities (RH) as low as 60%. Only a few days exposure to an inappropriate humidity may be enough for the decay process to be triggered in some specimens (for more information on the factors controlling the rate of pyrite decay, see Newman (1998)).