The mineral gardens of Mammoth Cave
Deep inside the dry upper passages of Kentucky's Mammoth Cave system, strange crystalline petals bloom from the limestone walls. These are not plants, but mineral formations called gypsum flowers. Each "flower" is a cluster of curving, fibrous crystals of calcium sulfate dihydrate (CaSO₄·2H₂O), the mineral known as gypsum. Unlike stalactites and stalagmites that form from dripping water, gypsum flowers emerge in arid sections of the cave where moisture slowly seeps through porous rock and evaporates.
The growth process begins when water percolates through the limestone, picking up calcium. This water also encounters sulfur-bearing minerals like pyrite (FeS₂) in the rock. Oxidation of pyrite creates sulfate ions. As this calcium- and sulfate-rich water reaches the cave passage, it evaporates. The dissolved calcium sulfate precipitates out of the solution, forming tiny gypsum crystals. The process is slow, occurring under capillary pressure as water moves through pores in the rock. The crystals grow from their base, being extruded or pushed outward from the limestone wall, which causes their characteristic curved, petal-like shapes.