Abstract: Zeophyllite, a rare calcium silicate, occurs at Monte Somma, Italy, as milky-white, slightly fibrous spherules associated with apophyllite on the surface of pumice in a welded tuff dated 472 ad). The chemical analysis and the indexed powder pattern, along with the chemical formula and unit cell dimensions calculated from these data, are given. The deficiency of Ca and Si cations and the abundance of (OH) and F anions in the chemical formula calculated on the basis of 38 anions (O + F) is interpreted as due to the presence of (O₄H₄) substituting for (SiO₄) tetrahedra and of fluorine instead of oxygen atoms in two anionic sites.