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 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.