Long-Term Stability of Evaporite Minerals: Geochronological Evidence

Abstract

The long term stability of evaporite minerals in areas being considered for the disposal of radioactive waste is critical in the overall assessment of such areas, especially when petrographic and other study indicate significant amounts of post-formational recrystallization. At the WIPP site in southeastern New Mexico, the age of the bedded evaporite minerals of the Castile and Salado Formations is known to be Late Permian, about 225 + 15 MYBP (millions of years before the present). It is possible, however, that the major accumulations of potassium salts were formed by diagenetic-epigenetic processes in the early Triassic as the section was buried, local brines caused remobilization of some salts, and several stages of potassium-bearing layers resulted. These processes were probably complete by about 200 MYBP (1). Important potassium ore minerals (note: here referred to as primary even though they may not completely have formed until about 200 MYBP) include sylvite, KC1 and langbeinite, K₂Mg₂(SO₄)₃; and minor amounts of carnallite, kainite, etc. are commonly interpreted to be primary. One of the most important secondary potassium-bearing minerals in polyhalite, K₇Ca₂Mg(SO₄)₄·2H₂0, because of its water content. Petrographic study clearly indicates that the polyhalite is secondary to sylvite, yet just how much younger has not been quantitatively established.