Lake Magadi is a saline soda lake in East African Rift Valley (Kenya). It is fed by perennial warm and hot saline springs. Na+-HCO3- type dilute inflows evolve into Lake Magadi brines rich in Na+, CO3 (2-), Cl-, HCO3- and SO4 (2-) and depleted in Ca2+ and Mg2+. The pH, CO3 (2-), and SiO2 content of these brines reach 11.5, 109000 ppm, and 1440 ppm respectively. Evaporative concentration coupled with mineral precipitation and fractional dissolution is thought to be the main process responsible for the stepwise evolution between dilute inflows and brines. In order to understand the details of the precipitation kinetics, we have performed simulations of mineral precipitation sequences and the resulting hydrochemical evolution during evaporation under different partial pressure of CO2 (pCO2) and temperature by using EQL-EVP program. In addition, we have performed laboratory precipitation experiments. The crystallization sequence was monitored by using in situ video microscopy and in situ and ex situ X-ray diffraction and Raman spectroscopy. The precipitation sequence was also monitored by scanning electron microscopy coupled with energy dispersive x-ray analysis. Trace amounts of magnesite, calcite, and pirssonite precipitate at the beginning. Magnesium silicate precipitate at low pCO2 (<-2.5) by redissolution of magnesite. Pirssonite forms from calcite dissolution at low pCO2. The rise in temperature highly delayed amorphous silica precipitation. Trona was the second precipitate. At low temperature-high pCO2, nahcolite precipitates at the second place whereas at high temperature-low pCO2, thermonatrite forms instead of trona. Halite is the third in the precipitation sequence. Burkeite (pCO2 of -3 to -4.5) and thenardite (pCO2 of -2 to -2.5) are the fourth in the sequence, which upon redissolution form glaserite. Sylvite, kalicinite, and villiaumite form at the end. Evaporation linearly raises the solute concentration until saturation of Na-CO3-HCO3 minerals and halite, which upon precipitation deplete solute content. Glaserite is a minor phase depleting K+ and SO4 (2-). The combination of modeling based on a kinetic approach and in situ mineralogical analysis is a powerful tool to understand mineral assemblages and kinetic precipitation pathways in soda lakes.

Chemical gardens are self-assembled tubular structures formed via abiotic precipitation upon the interaction of metal-ion salts with aqueous solutions of anionic species such as silicate, carbonate, or phosphate. These tubular structures have been suggested to be relevant for the early Earth and Earth-like planets and moons where alkaline silica and carbonate rich soda oceans are thought to be widespread. Carbonate and silica gardens are believed to be forming under the geochemical conditions of these soda oceans. Silica gardens are self-compartmentalized compositionally distinct bilayered mineral membranes. These membranes are small batteries that selectively catalyze the synthesis of prebiotically relevant organic compounds such as carboxylic acids, amino acids, and nucleobases by condensation of formamide. Recently, we have grown chemical gardens and mineral vesicles by immersing different metal salts in soda lake water and inferred that mineral self-organization could be a plausible scenario on soda oceans of early Earth and extraterrestrial planets and moons such as Enceladus. In this work, we have performed in-situ monitoring of the chemical gradient, pH and electrochemical potential differences across macroscopic calcium carbonate tubular structures grown by immersing calcium chloride salt pellets in carbonate-rich soda lake water (Lake Magadi, Southern Kenyan Rift Valley). To understand the temporal evolution of the growth process, we have performed ex-situ X-ray diffraction, Raman and infrared spectroscopy, and scanning electron microscopy of the tubular structures isolated after different periods of growth. We have also compared our results with calcium carbonate and silica gardens grown in model laboratory solutions. The walls of calcium carbonate tubular structures are composed of bilayers of texturally different but mineralogically similar crystalline calcite. We have observed that pH gradients across these “natural” calcium carbonate tubes are comparable to that of silica gardens and higher than that of synthetic carbonate gardens. We have discussed the implications of the texture, ionic gradient, pH and electrochemical potential differences across the tubes to material sciences, prebiotic chemistry, and origin of life.