A trick of crystallography
Deep beneath the limestone plateau of Apulia, the Castellana Caves show the area's mineralogy. Most cave formations, or speleothems, are made of calcite. Here, however, many are composed of aragonite. Both minerals share the identical chemical formula, CaCO₃, but differ in their crystal structure. Calcite has a trigonal crystal system, while aragonite's is orthorhombic. Under the low-pressure conditions found in caves, calcite is the more stable form. Aragonite shows specific chemical conditions in the groundwater that created a loophole in the laws of chemistry.
The reason is the local geology of the Murge plateau, which is full of Cretaceous limestone. Water percolating through these layers dissolves calcium carbonate, but it also picks up a significant concentration of magnesium ions (Mg²⁺). These magnesium ions effectively "poison" the formation of calcite crystals. They interfere with the calcite crystal lattice, inhibiting its growth. This suppression allows the less-stable aragonite, whose structure is not affected by the magnesium, to precipitate from the mineral-rich water instead. This process results in distinct formations. Instead of the massive, often opaque structures of typical calcite stalactites, the aragonite grows into needle-like (acicular) crystals, translucent spires, and complex bushes of "frostwork". The most spectacular display of these pure white aragonite and alabaster formations is in the Grotta Bianca, or White Cave.