East African magadi (trona): fluoride concentration and mineralogical composition

Abstract

Magadi from Lake Magadi, Kenya, Lake Natron, Tanzania, Lake Katwe, Uganda, El-Atrun, Sudan and efflorescent crust from the soil surface (scooped magadi) from northern Tanzania have been analysed chemically to determine fluoride and carbonates concentrations and by X-ray diffraction to determine the mineralogical composition. Magadi from Lake Natron and Lake Magadi are found to be very similar consisting mainly of trona (CO 3 2− + HCO 3 − > 10.4 meq [g magadi] −1) mixed with halite and either kogarkoite or villaumite, respectively, resulting in fluoride concentrations up to 8.7 mg F − [g magadi] −1. The scooped magadi is not as pure with respect to trona as the crystalline magadi, but the fluoride content is of same order of magnitude (0.23–5.1 mg F − [g magadi] −1). The scooped magadi consists of trona (CO 3 2− + HCO 3 = 3.5–9.5 meq [g magadi] −1) with different mixtures of halite, quartz, villiaumite, kogarkoite and thermonatrite. No fluoride containing minerals are identified in magadi from Uganda and Sudan, probably due to the very low fluoride concentrations of 0.02 and < O.24 mg (g magadi) −1, respectively, indicating that these samples are not contaminated with fluoride. The Sudanese magadi is a different mixture of trona, halite and quartz resulting in a variation in the carbonate concentration of 4.6–11.9 meq (g magadi) −1. The magadi from Lake Katwe consists of trona (CO 3 2− + HCO 3 − = 7.0 meq [g magadi] −1) mixed with burkeite and halite.