How do gypsum and thenardite form in evaporating playa brines?

In the context of playa geology, the formation of gypsum and thenardite is governed by the progressive evaporation of brines, which creates a zoned distribution of saline minerals similar to bathtub rings.1

As evaporation proceeds, the brine concentrates and precipitates minerals based on their solubility. After the initial deposition of carbonates, the process moves to the precipitation of sulfates. The specific mineral that forms depends on the chemical composition of the available brine:1

• Gypsum ($CaS{O}_4·2H_{2}O$): This mineral forms when there is a sufficient concentration of calcium present in the evaporating brine.1
• Thenardite ($N{a}_{2}S{O}_4$): If the brine contains less calcium, the sulfate component instead precipitates as thenardite, often alongside sodium carbonate.1

These sulfate-rich deposits typically form in a ring located inward from the outer margin of the salt pan, situated between the less-soluble carbonates and the highly soluble halite found at the center.1