Carbonate precipitation into alkaline silica-rich environments

Abstract

The precipitation of barium and strontium carbonates in silica-rich alkaline brines results in the formation of crystal aggregates that have noncrystallographic point symmetry currently displayed by biological organisms. The required silica concentration (as low as 250 ppm SiO 2 ) is within the levels reported from some contemporary alkaline lakes. The striking morphological behavior of these inorganic precipitates and the specific range of pH values (8.5–12) within which they form make them useful pH indicators for past environments. The discovery of similar structures in Archean cherts, which are thought to form via silica gels precipitated from alkaline silica-rich waters, could provide direct information on the chemistry of the primitive waters and are useful to decipher the actual origin of some Precambrian microstructures currently interpreted as biological remnants.