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I am doing a PhD about archaeometry in order to study the contaminations and alterations that occur to the ceramics which are buried in a marine environment. In my research, I have found pyrite in the cracks of my ceramics. Now, I am trying to find jarosite.

Well, I have found bibliography about pyrite's oxidation reaction to give jarosite (Secco et al. [1]). Now, trying to understand the redox reaction of pyrite to jarosite, I am getting blocked. The image of the reaction is the next:

The presence of jarosite and absence of pyrite in the outermost part of the margin in the Olbia potsherds is due to the oxidation of pyrite and reaction with dissolved alkali (Faure 1991), according to the following reactions (Faure 1991):

$$ \begin{align} \ce{FeS2(s) + 7/2 O2 + H2O &-> Fe^2+ + 2 SO4^2- + 2 H+};\tag{18}\\ \ce{Fe^2+ + 1/4 O2 + H+ &-> Fe^3+ + 1/2 H2O};\tag{19}\\ \ce{Fe^3+(aq) + 3 H2O &-> Fe(OH)3(s) + 3 H+};\tag{20}\\ \ce{(K+{,}Na+) + 3 Fe(OH)3 + 2 SO4^2- + 3 H+ &-> (K{,}Na)Fe3(SO4)2(OH)6 + 3 H2O}.\tag{21} \end{align} $$

I would like to understand the half redox reactions (oxidation and reduction). I have tried to solve this, but I have not been able to. I identify the reducing agent (the S, which oxidizes from S(1-) to S(6+)), and the oxidizing agent (the oxygen, which reduces from O(0) to O(2-). However, I think I am unable to write the half reactions properly, because I do not obtain the same as the equation (18) of the bibliography:

enter image description here

What I obtain is that the second half reaction also seems an oxidation. I can't solve it like a reduction.

I will appreciate a lot your help, maybe you can shed a little bit of light on this.

References

  1. Secco, M.; Maritan, L.; Mazzoli, C.; Lampronti, G. I.; Zorzi, F.; Nodari, L.; Russo, U.; Mattioli, S. P. Alteration Processes of Pottery in Lagoon-like Environments. Archaeometry 2011, 53 (4), 809–829. https://doi.org/10/cp6gkh.
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  • $\begingroup$ Oxygen is the oxidant $\endgroup$ – Waylander Sep 9 at 13:59
  • $\begingroup$ Yes, thank you! I am trying to solve the reaction of the reduction, but I can't. What I get is as if the second reaction was another oxidation. $\endgroup$ – uxusan Sep 9 at 15:13
  • $\begingroup$ The second reaction is also an oxidation with oxygen as the oxidant $\endgroup$ – Waylander Sep 9 at 15:21
  • $\begingroup$ So, I understand that it is not a redox reaction? Only oxidation? Sorry, I think I am missing some concepts. And how can I obtain the same as equation (18)? Thank you $\endgroup$ – uxusan Sep 9 at 15:25
  • $\begingroup$ What I don't understand is the statement "due to the oxidation of pyrite and reaction with dissolved alkali" when none of the processes quoted have OH- in the equation. The process requires only water and oxygen. $\endgroup$ – Waylander Sep 9 at 15:56

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