By Aodhán Ó Gogáin, Documentation Officer, National Museum of Ireland
Museum collections normally contain delicate and fragile objects, from old, degraded manuscripts, and now-corroded and brittle metal works to ancient ceramics. Rocks within a museum, however, would generally be considered the “more robust” elements of a collection, and to a large extent this is true. But, with the exception of falls and breaks, there are other hazards to rock collections that, over time, can disintegrate specimens within otherwise secure and stable storage areas.
Pyrite Disease
Pyrite, and its mineral cousin marcasite, is a mineral which can be found in a variety of rocks, including limestones and mudstones, and can occur in association with other minerals. Exposure to humid air can cause pyrite to react with oxygen and water, resulting in new minerals. One problem is that these new minerals expand, cracking and fragmenting the specimen, while also coating affected areas in hydrated iron sulfides (Fig. 1). This disintegration process is called pyrite disease. Sulfuric acid is produced as a side product, which damages anything it comes into contact with, from labels and containers to other specimens. Once pyrite disease sets in, any damage to the specimen is near permanent: there is no way to “cure” the already “diseased” part. If left unchecked, pyrite disease can lead to the complete disintegration of a specimen. Pyrite disease can be mitigated and halted by housing affected specimens in low oxygen and low humidity environments.
Absorbent minerals
Halite or “rock salt” absorbs moisture from the air, which has two effects. Firstly, the moisture causes the halite to dissolve, gradually breaking the specimens down. Secondly, as halite pulls moisture from the air, it causes water to pool around the specimen, soaking labels and flooding containers and drawers in a nasty brine. To inhibit this, halite needs to be stored in a low humidity environment. A simple way to do this is to seal the specimen in a box that contains silica gel (Fig. 2).
Unstable minerals and rocks
Not all minerals are stable at surface pressure and when surrounded by atmospheric gases. The mineral olivine, which forms deep in Earth’s crust (e.g., in peridotite rock), when brought to the surface can undergo depressurized alteration and disintegration. The water-rich mineral borax (Fig. 3) will dehydrate when exposed to only moderate ambient heat, causing the specimen to crumble into a fine powder. And some minerals (e.g., proustite, realgar) need to be kept in the dark: simply exposing them to light will ruin them.
Conclusion
Although rocks and minerals are generally stable and require limited care compared to more fragile museum objects, they are not without hazards. Without prior knowledge of the dangers to them and proper care, a fine museum specimen can quickly become a pile of dust.Visit the Dead Zoo Lab at Collins’ Barracks to see some of our mineral and rock collection.
