The identification is described of a white substance found in a wooden box as part of the cargo of a sunken Roman ship of the 2nd century A.C. discovered in the Gulf of Procchio on the north coast of Elba in 1966. The sample was identified as huntite, CaMg₃(CO₃)₄, on the basis of chemical analysis, thermo-gravimetric analysis, differential thermal analysis, x-ray diffraction and electron microscopy. Formation by the reaction of sea water on some pre-existing material is ruled out. -- AATA