Cation exchanged Fe(II) and Sr compared to other divalent cations (Ca,Mg) in the bure Callovian-Oxfordian formation: Implications for porewater composition modelling

摘要: Abstract Iron and Sr bearing phases were thoroughly investigated by means of spectrometric microscopic techniques in Callovian–Oxfordian (COX) samples originating from the ANDRA Underground Research Laboratory (URL) Bure (France). Strontium was found to be essentially associated with celestite, whereas Fe distributed over a wide range mineral phases. mainly present as Fe(II) studied (∼93% Mossbauer results). Most pyrite, sideroplesite/ankerite clay minerals. Iron(III), if present, minerals (probably illite, illite-smectite mixed layer chlorite). No Fe(III) oxy(hydro)xide could detected samples. Strontianite not observed either. Based on these observations, it is likely that COX porewater equilibrium following carbonate minerals, calcite, dolomite ankerite/sideroplesite, but strontianite. It shown this information can combined cation exchange composition order give direct estimates or constraints solubility products dolomite, siderite As consequence, an experimental method developed retrieve exchanged very well preserved The homogeneous formation completely agreement presence calcite whose reactions control part composition. Amongst broad values available for thermodynamic databases, value log Kdolomite = −3.57 most appropriate one formation. With regard strontianite, appears constant tabulated Llnl database valid natural systems. given Busenberg et al. used other databases seems far more appropriate. Concerning siderite/ankerite equilibrium, measured Fe/Ca ratio exchanger (∼0.01) only considered maximum due possible bias, leading constraint sideroplesite phase COX: Fe0.7Mg0.2Ca0.1CO3 + H+ ⇔ 0.7 Fe2+ + 0.2 Mg2+ + 0.1 Ca2+ +  HCO 3 - logKsid_COX  This estimated based solid solution model: log Ksid_COX = 0.23.