The iron disulphide minerals include ferroselite, melnikovite, marcasite and pyrite.' The first two minerals are rare sulphides and are described in Chapter 5. This chapter deals with marcasite and more especially pyrite, because they undoubtedly contribute to the majority of conservation problems found in geological collections. Pyrite is one of the commonest sulphides, occurring across the geological spectrum; marcasite occurs less frequently but causes similar problems.