Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Vladimír Žáček, Vladislav Rapprich, Jiří Šíma, Radek Škoda, František Laufek, Firdawok Legesa

Kogarkoite, Na₃(SO₄)F, from the Shalo hot spring, Main Ethiopian Rift: implications for F-enrichment of thermal groundwater related to alkaline silicic volcanic rocks

Journal of Geosciences, volume 60 (2015), issue 3, 171 - 179

DOI: http://doi.org/10.3190/jgeosci.195

Kogarkoite, anhydrous monoclinic sodium fluorosulphate, was identified as a white granular coagulate in the Shalo hot spring situated between the towns of Hawasa and Shashemene, in the central part of the Main Ethiopian Rift in southern Ethiopia. The mineral crystallizes together with trona, Na₃(HCO₃)(CO₃)·2H₂O, and opal as white stratified sinter covering the surface of rhyolite chips semi-immerged in Na-HCO₃ type thermal water with temperatures of up to 96 °C. The determined chemical composition of kogarkoite is close to the ideal formula Na₃(SO₄)F and the refined unit-cell parameters are a = 18.089(2) Å, b = 6.965(1) Å, c = 11.457(1) Å, β = 107.72(1°), V = 1374.4(4) ų.
Increased concentrations of fluorine in groundwater were detected in the area of the rift floor of the central and northern parts of the Main Ethiopian Rift including the wider vicinity of the towns of Shashemene and Hawasa; however, this is the first described occurrence of fluorine-bearing mineral related to hot springs in Ethiopia. The presence of “free” fluorine both in cold groundwater (up to 17 mg/l) and in the Shalo hot spring (40 mg/l) is related to the prevalence of highly alkaline silicic volcanic rocks (namely pantelleritic obsidians) extremely poor in calcium and phosphorus. This prevents fixing of fluorine in apatite or in other stable minerals.