The sulfide minerals with an asterisk (*) are arranged sequentially in the order in which they can form in soils.
Minerals without an asterisk are metastable in soils and normally do not form. The metastable sulfides are shown in Fig. 17.4 by short dashed lines.
The metastable lines lie below the stable sulfide for that element. If near equilibrium conditions are maintained, the redox cannot drop below a given line until the transformation occurring at that line is complete. When complete, the initial solid phase is dissolved and the metal sulfide remains to control the solubility relationships of that element. The metastable sulfides are only
included to show the redox at which they would have formed had the initial phase governing that cation remained. In some cases the metastable sulfide becomes the stable phase at lower redox (for example see HgS(cinnabar)
in Fig. 2l.9).
The sequence of metal sulfides that form in soils and the pe + pH at which they precipitate are summarized in Table 17.4. The first-formed sulfide is Hg2S(c) and the least likely to form is MgS(c). As pointed out in Segtion ]g7_2
and 17.3 S(rhombic) normally does not form in soils because the low redox that is required raises the partial pressure of H2S(g) sufiiciently that it