MODERN surface sediments containing H2S are often coloured black by fine grained, iron monosulphide minerals such as mackinawite, Fe1+xS, and greigite, Fe3S4. The black monosulphides are thermodynamically unstable, under H2S-rich sedimentary conditions, relative to the disulphide pyrite, FeS21,2, which is not a black pigmenting agent. As a result, the black colour normally disappears with depth during diagenetic transformation to pyrite2,3. In some situations, however, such as in older buried layers of the deep water sediments of the Black Sea, the black monosulphides persist.