A proposed fog deposition mechanism for the formation of salt efflorescences in the Mpumalanga highveld, Republic of South Africa

White efflorescences, representing evaporites that coat stubble grass vegetation, were observed in September 1991 on a farm in the Ermelo area, Mpumalanga highveld of the Republic of South Africa. Occurrences were in irregular patches. Investigation of the evaporites by means of X-ray diffraction. differential thermal analysis, scanning electron microscopy with EDX, and chemical analyses revealed the following minerals, presented in order of decreasing abundance: thenardite, bloedite, gypsum and kieserite. Accumulation of the salts, which are highly water soluble, probably occurred during the six dry winter months, suggesting a deposition rate of up to 11-15 g of thenardite m(-2) yr(-1). It is proposed that the efflorescences had formed by fog impaction on grass blades. They are, to the authors' knowledge, the first recorded evaporite deposits formed by atmospheric deposition.