580 MINERALS/Sulphides Figure Stability relations of iron sulphides and oxides in water at 25 C and atmosphere total pressure (A) at a total activity of dissolved sulphur of 10 and (B) at a total activity of dissolved sulphur of 10 only a very small stability field The effect of lowering the total sulphur activity to 10 is shown in Figure 4B: the pyrite field is now much smaller and the magnetite field has increased in size Notable from examination of Figure is the very large stability field of pyrite relative to pyrrhotite in sediments It can also be seen that quite low sulphide activities can stabilize pyrite over a considerable range of Eh conditions, and that iron oxides or carbonate will form only when the sulphide activity of the environment is very small The problem of metastability is implicit in the study of iron sulphides For example, certain iron sulphides that are common in nature, such as marcasite (FeS2) and mackinawite (Fe1ỵxS), are not included in the phase diagrams of Figure These sulphides cannot be synthesized by a direct reaction between iron and sulphur in dry systems but only by precipitation from solution In the natural aqueous systems where such iron sulphides form, reduced sulphur is readily available from the bacterial reduction of sulphate Here mackinawite is the dominant precursor phase, with the ultimate stable product being pyrite Details of the reaction pathways involving mackinawite and other less common precursors are still being established, but Figure makes an attempt to summarize known and more speculative phase transformations When interpreting the mineral assemblages and textures in order to understand the genesis of sulphide Figure Summary of known and speculative phase transform ations and reactions in the low temperature iron sulphur system (redrawn after Lennie and Vaughan, Geochem soc spec pubh No 5, p.128, 1996) The letters on the diagram are as follows: A, formation from Fe2ỵ or Fe and HS or H2S at a pH of (T < 523 K); E, inferred sulphidation of greigite at a pH of more than (T < 523 K); F, solid state transformation; G, sulphidation of Fe by H2S at a pH of 10 (T < 523 K); I, ageing of amorphous precipitate; J, solid state transformation, kinetics not established; L, precipitation from Fe2ỵ by HS at a pH of (wide range of temperatures to above 573 K); M,N, reaction of Fe2ỵ and HS or H2S at a pH of (or speculative transformation of proposed wurtzite FeS structure) to form either (M) hexagonal pyrrhotite (T > 413 K) or (N) troilite (T < 413 K); O, marcasite after pyrrhotite; P, pyrite formed by sulphidation of pyrrhotite (this reaction is rapid above 573 K); Q, troilite hexagonal pyrrhotite reversible phase transition Reproduced with permission from Geochemis try of Hydrothermal Ore Deposits, ed H L Barnes, 1997, John Wiley and Sons