EXPANSION OF THE HARDIE-EUGSTER BRINE EVOLUTION MODEL INTO THE LOW PH REALM

The Hardie-Eugster (1970, 1978) “chemical divides” model for tracing brine evolution and resulting evaporite minerals revolutionized how the scientific community regarded the acquisition of solutes by surface water-mineral interactions. This work incorporated case studies from a wide diversity of neutral – alkaline brine environments. A recent modification, the Long-Lyons-Hines (2009) model, introduced a low pH pathway for the first time by documenting moderately acid brines and their minerals at Lake Tyrrell in Victoria, Australia. We propose that there is a diversity of acid brines and that much work is needed to complete the range of brine evolution that exists on Earth, and perhaps on other planets.

Our recent and continuing studies of acid brine ephemeral lakes in Western Australia and Chile show dynamic physical and chemical processes at individual lakes, as well as from lake to lake. The most extreme modern brines observed have pH as low as 1.4 and salinity as high as 32% TDS. Modeling suggests that water activity may be as low as 0.57 for highly evaporated acid brines. There is great temporal heterogeneity in lake waters and spatial heterogeneity in groundwaters. Because flooding and evapoconcentration change lake water pH and salinity, and because many of the chemical sediments are pH-sensitive, complex water composition exists and abundant mineral precipitation and dissolution occurs. In fact, an expansion of the model necessitates the addition of minerals such as some metal sulfate salts that form only in acids. The mineral suite formed at acid brine lakes may include halite, gypsum, bassanite, basaluminite, hematite, jarosite, alunite, rozenite, kaolinite/halloysite, native sulfur, and opalline silica. Traditional brine ternary diagrams based on Ca²⁺, Mg²⁺, SO₄^2-, and HCO₃^- may need to be replaced for brines with no measurable HCO₃^-, multiple S species, and concentrations of Al³⁺, Si⁴⁺, and/or Fe^2+/3+ higher than Ca²⁺ and Mg²⁺. Detailed studies of individual modern acid brine lakes are needed to complete the low pH end of the brine evolution model.