Ni‑rich mineral nepouite explains the exceptional green color of speleothems

Speleothems are secondary mineral structures typically found in karstic caves and usually composedof calcite or aragonite. Despite being naturally white, some might exhibit unusual colors, such asblue, black, red, yellow or green. The causes of these exceptional colorations are poorly understood,especially for green speleothems, which are barely reported. Here we describe the occurrence of thegreen Ni‑bearing serpentine nepouite in green aragonite and calcite speleothems, in the Aven duMarcou (Hérault, France). Nepouite is mainly found as flat lamellar crystals in the outer rim of greenspeleothems and crystallized alongside radially grown aragonite crystals. This supports nepouitebeginning to crystallize recently, due to a change in the chemical composition of the water. Nepouitealso exhibits extensive substitution between Ni, Mg and Zn. The various elements responsible fornepouite precipitation are thought to come from the weathering of pyrite crystals in the overlyingrocks, which is consistent with the pH conditions of the cave and the Al‑free composition of nepouite.This study explains the crystallization mechanisms and stability conditions of silicate minerals incolored caves.