CARBONATE PRECIPITATION INTO ALKALINE SILICA-RICH ENVIRONMENTS

The precipitation of barium and strontium carbonates in silica-rich al kaline 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 SiO2) is within the levels reported from some contemporary alkaline lakes, T he 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 simi lar structures in Archean cherts, which are thought to form via silica gels precipitated from alkaline silica-rich waters, could provide dir ect information on the chemistry of the primitive waters and are usefu l to decipher the actual origin of some Precambrian microstructures cu rrently interpreted as biological remnants.