Experimental precipitation and genesis of sepiolite at earth-surface conditions

Reaction of aqueous silica with sea water produces a hydrated magnesium silicate similar to sepiolite in structure and composition. Our investigations indicate that sepiolite is the only non-aluminous, cation-bearing silicate that can be precipitated directly from sea water. Experiments with magnesium-free sea water and with magnesium chloride solutions show that cations other than Mg²⁺ are not involved in the precipitation of this compound. The equilibrium constant for the reaction: 2Mg²⁺+3SiO_2aq+(n+2)H₂O = Mg₂Si₃O₈(H₂O)_n+4H⁺ has been determined experimentally at 25°C and one atmosphere total pressure and the ΔG°_֬ of dehydrated sepiolite calculated to be -903.3 ±0.5 kcal/mole. Stability lelations in the system MgO-SiO₂-H₂O at 25°C and one atmosphere total pressure are examined and models for the genesis of sepiolite in the marine environment, the saline lake environment and by chemical weathering of ultramafic rocks are discussed.