Cretaceous evaporites, consisting of a thick halite facies and economically valuable deposits of sylvite, carnallite, and tachyhydrite (CaCl2 · MgCl2 · 10H2O), are present in the Sergipe basin along the northeast coast of Brazil. In the Japaratuba subbasin at least 2 carnallite deposits, 2 tachyhydrite deposits, and 1 sylvite deposit have been outlined by drilling by the Brazilian government. Carnallite and sylvite have also been found in the adjoining Siriri and Treme subbasins. The Sergipe deposits formed in a small embayment connected to a larger evaporite basin which developed in Aptian time during the initial detachment of the South American and African continents.
The Sergipe evaporites are unique in that they are noticeably deficient in both carbonate and sulfate minerals, and contain thick almost pure beds of the rare mineral tachyhydrite. The tachyhydrite appears to be part of a primary depositional sequence which, starting with the least soluble salt, is (1) halite, (2) carnallite, and (3) tachyhydrite. This sequence required a brine enriched in calcium chloride and a super-efficient desiccation system to saturate what was probably an extremely hygroscopic brine. Mixing of incoming sulfate-laden seawater and calcium chloride in the brine, which resulted in precipitation of calcium sulfate outside the basin, may account for the sulfate deficiency. The enrichment of the evaporite brine in calcium chloride may be the result of reaction between magnesium chloride in the original brine and calcium carbonate in the incoming seawater and previously deposited carbonates. This reaction would also explain the lack of carbonate minerals in the evaporites.
The sylvite deposits in the uppermost evaporite cycle occur at depths between 300 and 700 m and seem to be most favorable for exploitation. Carnallite deposits in 2 older cycles are also potential sources of potash and magnesium. In addition the tachyhydrite deposits constitute a new and potentially valuable source of magnesium. The upper deposit is locally more than 100 m thick and contains an estimated 1.5 billion metric tons of magnesium chloride. Preliminary studies suggest that the tachyhydrite deposits have certain advantages over seawater as a magnesium chloride source.