Gypsum Caves and Selenite Crystals
Most caves form in limestone and grow calcium-carbonate formations. A smaller but spectacular subset forms in or near gypsum (CaSO4·2H2O) and grows selenite crystals — sometimes of extraordinary size.
How gypsum caves form
Gypsum dissolves much more readily than limestone — in some conditions 100 times faster. Pure gypsum caves form quickly and collapse equally fast. The longer caves form in mixed limestone-gypsum sequences where slower limestone dissolution preserves passage geometry.
Selenite crystals
Selenite is the transparent crystal form of gypsum. It can grow to enormous size when water chemistry remains stable: the Cave of Crystals at Naica produced crystals 11 m long. Lechuguilla in New Mexico contains gypsum chandeliers up to 6 m long.
Sulfuric acid speleogenesis
The Guadalupe caves (Lechuguilla, Carlsbad, Slaughter Canyon) formed by sulfuric acid dissolution — H2S from petroleum deposits below reacted with oxygenated groundwater. The acid dissolves the limestone and leaves gypsum as a byproduct, which accumulates as massive deposits.
Other gypsum caves
Ukraine's Optymistychna is the world's longest gypsum cave at 260+ km, formed in Miocene gypsum. The Black Hills of South Dakota contain extensive gypsum-rich caves. Sicily's gypsum karst includes the Cattolica gypsum caves.
Why crystals matter
Slow, stable mineral precipitation conditions produce crystals impossible under faster or more dynamic conditions. Studying gypsum cave crystals reveals geological timescales of stability that surface environments cannot match.
Visitor access
Naica is closed (filling with water since 2017). Lechuguilla is research-only. Optymistychna requires Ukrainian club access. Most gypsum-rich caves are restricted. Smaller gypsum-decorated caves in Sicily and South Dakota are publicly accessible.
Explore on the map
See gypsum caves on the interactive map.