Abstract
Eh, pH and log activity diagrams were calculated to demonstrate the equilibrium relationships between jarosite-goethite-pyrite at 25 °C, 1 atm. Interpretation of these diagrams in light of natural occurrences of iron minerals in the oxidation zone of a sulfide deposit indicates that goethite (or hematite) is the stable mineral in the normal weathering environment. Jarosite is shown to be stable in an acid (pH<3) medium which is moderate to highly oxidizing (Eh-pH). It appears that the persistence of jarosite outside its stability field is due to the sluggishness of the reaction jarosite → goethite.
Resume
Les diagrammes d'activité chemique logarithmique, de Eh et de pH ont été calcules pour démontrer les rapports entre jarosite-goethite-pyrite à 25 °C, 1 atm. L'interprétation de ces diagrammes à la lumière des occurences naturelles de minéraux ferrifères dans la zone oxydée d'un dépôt de sulfures indique que la goethite est le minéral stable dans un environnement d'altération surficielle. On montre que la jarosite est stable en milieu acide (pH<3) et oxydant (Eh-pH). La persistance de jarosite en dehors de son champ de stabilité peut être attribuée à la lenteut de la réaction jarosite → goethite.
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Brown, J.B. Jarosite-geoethite stabilities at 25 °C, 1 ATM. Mineral. Deposita 6, 245–252 (1971). https://doi.org/10.1007/BF00208032
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DOI: https://doi.org/10.1007/BF00208032