Jarosite growth zoning as a recorder of fluid evolution

Growth zoning in jarosite, an Fe³⁺-bearing sulfate mineral, can generally be characterized by variation of Na⁺ for K⁺ in the crystallographic A site or by Al³⁺ for Fe³⁺ in the octahedral (B) site. Growth zoning in a sample from Post Pit, NV, however, is more complicated than has been observed in other jarosite samples examined in this study and by previous work, and is characterized by varying Ba, Sr, P, and As. In this sample, these elements define a coupled substitution in which Ba²⁺ and Sr²⁺ substitute for K⁺ in the crystallographic A site, and are balanced by the substitution of P⁵⁺ and As⁵⁺ for S⁶⁺ in the tetrahedral (X) site. The Post Pit sample also exhibits a high concentration of V, which does not appear to participate in the aforementioned coupled substitution. Analysis of mineral stoichiometry and charge balance reveal that V is likely tetravalent, and represents a charge excess in the B site. The occurrence of V-bearing goethite in this sample records fluctuating fluid pH, resulting in the alternating stability of jarosite and goethite. Measurement of the REEs in jarosite by SIMS, indicate that Ce is trivalent in the Post Pit sample; reduced relative to Ce⁴⁺ found in the remainder of the sample suite. The presence of V⁴⁺ and Ce³⁺ indicate that the Post Pit sample was deposited in a less oxidizing environment than the remainder of the sample suite. This study illustrates how jarosite crystal chemistry can be used to place relative constraints on fluid conditions (pH, Eh, chemistry, etc.) in the terrestrial environment, and characterizes possible major and trace element coupled substitutions into the jarosite crystal structure. Furthermore, this study provides a framework for future studies to examine mineral-fluid partition coefficients (D values) which may place more absolute constraints on fluid chemistry in the jarosite depositional environment.