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+, CO32-,
Cl-, HCO3- and
SO42- and depleted in
Ca2+ and Mg2+. The pH,
CO32- 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 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 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 a minor phase
depleting K+ and
SO42-. 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.