Phillipsite is a mineral of the zeolite group; a hydrated potassium, calcium and aluminium silicate, approximating to (Ca,Na2,K2)3Al6Si10O32·12H2O. (Also with sodium replaced by calcium: KCaAl3Si5O16·6H2O). The crystals are monoclinic, but only complex cruciform twins are known, these being exactly like twins of harmotome. Crystals of phillipsite are, however, usually smaller and more transparent and glassy than those of harmotome. Spherical groups with a radially fibrous structure and bristled with crystals on the surface are not uncommon. The Mohs hardness is 4.5, and the specific gravity is 2.2. The species was established by A. Levy in 1825 and named after William Phillips. French authors use the name christianite (after Christian VIII of Denmark), given by A. Des Cloizeaux in 1847.
Phillipsite is a mineral of secondary origin, and occurs with other zeolites in the amygdaloidal cavities of mafic volcanic rocks: for example in the basalt of the Giants Causeway in County Antrim, and near Melbourne in Victoria; and in Lencitite near Rome. Small crystals of recent formation have been observed in the masonry of the hot baths at Plombires and Bourbonneles-Bains, in France. Minute spherical aggregates embedded in red clay were dredged by the Challenger from the bottom of the Central Pacific, where they had been formed by the decomposition of lava.