Deterioration

The usual environmental factors, light, heat, dust, relative humidity and pollutants affect rocks and minerals in similar ways to which they affect materials considered to be more sensitive. Certain minerals, realgar (AsS) and pyrargite (Ag₃SbS₃) for example, are so sensitive to light that they only retain their true colours if they are stored in the dark.

In addition to light-induced damage, deterioration mechanisms for rocks and minerals include:

• physical damage of fragile specimens due to abrasion and poor handling;
• dissolution of specimens by absorption of water from the environment;
• loss of water to the environment and subsequent changes in chemical composition and properties;
• corrosion of components;
• fracturing or volatilisation of crystals by exposure to heat or cycles of hot and cold;
• chemical attack by acidic pollutants released by certain wood types such as oak, birch and chipboard used in storage cabinets and attack by agents used in cleaning and other cosmetic treatments;
• biological attack and chemical degradation under conditions of high relative humidity; and
• deterioration by radioactive decay.

Dust is potentially very damaging to minerals and rocks. In addition to disfiguring surfaces dust encourages corrosion and other reactions by providing nucleation sites for the absorption of water and pollutants from the air.

One of the problems associated with collections of rocks and minerals is the presence of impurities in some specimens. It has been suggested that the presence of impurities may account for the different behaviours of supposedly identical minerals exposed to the same conditions. Some brown topaz specimens for example, fade in light while others are stable (Nassau 1992). Thus, although guidelines can be given for the care of rock and mineral collections, these may be compromised by impurities which lead to the deterioration of some specimens.

Note that while some deterioration processes are rapid, others may only become evident after many years.