THE FORMATION AND GROWTH OF PYRITE, MARCASITE, AND CUBIC FERROUS SULFIDE

Part I. Pyrite (py) and marcasite (mc) were synthesized from aqueous solutions at 75(DEGREES)C. The pH-dependent boundary between py and mc precipitation was found to correlate with pK(,1) of the dominant H(,2)S(,n) (2 < n < 6) species present, with mc forming at pH < pK(,1) and py at pH > pK(,1). Hence, mc forms below pH = 5.0 when H(,2)S(,2) (pK(,1) = 5.0) is the dominant polysulfide and below pH = 3.9 when H(,2)S(,4) (pK(,1) = 3.9) is dominant. Only py is formed above pH = pK(,1) in the same cases.; Protons play a structure-determining role in the precipitation mechanism by tying up 1(pi)(,g)('*) electrons in the disulfide group, promoting distortion of the Fe coordination around the S atoms to a mc-like configuration. The nonprotonated ends of HS(,n)('-) and S(,n)('2-) do not cause that distortion, allowing the pyrite structure to form. The correlation with pK(,1) is due to electrostatic effects: the protonated ends of H(,2)S(,n) and HS(,n)('-) are repelled from the positively-charged growth sites in py, but the nonprotonated S end of HS(,n)('-) and S(,n)('2-) are attracted into those sites. Diprotonated H(,2)S(,2) can easily attach to the growing surfaces of mc, though.; Marcasite occurrence in nature indicates that (1) the pH was less than 5.0, (2) the temperature was less than 150(DEGREES)C, and (3) formation of H(,2)S(,2) occurred at the site of deposition. The polysulfide species probably form by partial oxidation of H(,2)S or by reaction of H(,2)S and more-oxidized S species such as S(,2)O(,3)('2-).; Part II. Pyrite crystals were synthesized by hydrothermal transport using 5m NH(,4)Cl in sealed glass capsules at 250(DEGREES) to 500(DEGREES)C. The morphology of the crystals varies from cubic to octahedral to pyritohedral with increasing temperature and degree of supersaturation (S). The growth mechanism changes from screw-dislocation growth at T < 300(DEGREES)C and very low S to layer-by-layer growth at higher T and S. Extremely high S during quenching led to diffusion-controlled rather than surface-controlled growth reactions which produced dendritic morphologies.; Part III. Metastable cubic FeS (sphalerite structure) was synthesized by the aqueous sulfidation of Fe powder. Cubic FeS formed below 92(DEGREES)C at pH < 6.0 and was associated with troilite and mackinawite.