Mineral Precipitation from Soda Brines, Lake Magadi, Rift Valley (Kenya)

Abstract:

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 are thought to be the main processes 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 pCO_2, thermonatrite forms instead of trona. Halite is the third in 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 forms 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 the major phase depleting K⁺ and SO₄^2-. The combination of modelling 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.