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

Magadi from Lake Magadi, Kenya, Lake Natron, Tanzania, Lake Katwe, Uganda, El-Atrun, Sudan and efflorescent crust from the soil surface (scooped magad i) from northern Tanzania have been analysed chemically to determine fluori de and carbonates concentrations and by X-ray diffraction to determine the mineralogical composition. Magadi from Lake Natron and Lake Magadi are foun d to be very similar consisting mainly of trona (CO32- + HCO3- > 10.4 meq [ g magadi](-1)) mixed with halite and either kogarkoite or villiaumite, resp ectively, 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 crystall ine magadi, but the fluoride content is of the same order of magnitude (0.2 3-5.1 mg F- [g magadi](-1)). The scooped magadi consists of trona (CO32- HCO3 = 3.5-9.5 meq [g magadi](-1)) with different mixtures of halite, quart z, villiaumite, kogarkoite and thermonatrite. No fluoride containing minera ls are identified in magadi from Uganda and Sudan, probably due to the very low fluoride concentrations of 0.02 and <0.24 mg (g magadi)(-1), respectiv ely, indicating that these samples are not contaminated with fluoride. The Sudanese magadi is a different mixture of trona, halite and quartz resultin g in a variation in the carbonate concentration of 4.6-11.9 meq (g magadi)( -1). The magadi from Lake Katwe consists of trona (CO32- + HCO3- = 7.0 meq [g magadi](-1)) mixed with burkeite and halite. (C) 1999 Elsevier Science L imited. All rights reserved.