SEARCH

SEARCH BY CITATION

ABSTRACT

Pseudohexagonal aragonite crystals are common components in some hot-spring travertines at Chemurkeu on the western shore of Lake Bogoria, Kenya. Beds, lenses and pods of aragonite crystals are intercalated with beds of white non-crystallographic calcite dendrites. The pseudohexagonal aragonite crystals, which are up to 4 cm long and 4 mm wide, are formed of nested skeletal crystals. Each skeletal crystal is formed of cyclical twinned crystals that are constructed of stacked subcrystals. The latter are inclined at a consistent angle of 40° to the long axis of the pseudohexagonal aragonite crystal. Intense competition for space during growth modified the crystal morphology with the result that many of the pseudohexagonal crystals are distorted. Intercrystalline and intracrystalline pores are filled or partly filled by epitaxial aragonite overgrowths and/or reticulate microbial coatings that have a high concentration of Si and Mg. In places, this extracellular mucus induced etching of the underlying aragonite crystal.

Today the hot (T>95 °C) Na-HCO3-Cl spring waters at Chemurkeu have a salinity of 5–6 g L−1 TDS, a pH of 8·1–9·1, Ca2+ concentrations of <2 mg L−1 and Mg2+ concentrations of <0·7 mg L−1, The springs of the Lake Bogoria Geothermal Field are fed by a shallow aquifer (T∼100 °C) and a deeper aquifer (T∼170 °C). Springs at Chemurkeu derive from meteoric groundwater, lake water and condensed steam, and are fed mainly from the shallow thermal aquifer. Much of the aragonite may have formed when the spring waters contained more dissolved Ca2+ than today, possibly under more humid conditions during the Holocene.