Some of the most common minerals on Earth, and a few genuinely striking ones, owe their existence to something disappearing rather than something forming: water. Evaporite minerals are what’s left behind after a body of mineral-rich water evaporates away completely, concentrating dissolved salts until they finally crystallize out.
Concentration, not creation
Every evaporite mineral starts as ordinary dissolved ions in water, seawater most often, though closed desert basins and inland seas work the same way. As water evaporates faster than it’s replenished, the remaining water becomes increasingly concentrated, and different minerals crystallize out in a predictable sequence as their individual solubility limits are reached, the least soluble minerals first, the most soluble last. Halite, ordinary rock salt, and gypsum, the mineral behind selenite, are two of the most common results, but the same process also concentrates rarer elements in specific settings, which is why some evaporite basins become economically important sources of potassium, boron, or lithium.
A record of ancient climate
A thick evaporite deposit is direct physical evidence that a specific location was, at some point in its geologic past, an isolated basin in an arid climate, evaporating faster than it was being refilled, sometimes over enormous timescales. Finding evaporite layers deep in a rock sequence tells geologists something real about ancient climate and geography in a way few other rock types can, a dried-up ancient sea leaves behind a mineral record even after every trace of the water itself is long gone.
It’s a useful contrast to most of the minerals in this shop, which formed through addition, magma cooling, hydrothermal fluid depositing new material. Evaporites are a reminder that subtraction, water simply leaving, is just as capable of leaving crystals behind.
