PHILLIPSITE, a mineral of the zeolite group; a hydrated potassium, calcium and aluminium silicate, approximating to (K2, Ca)Al2(SiO3)4·4H2O. It varies somewhat in composition, and a variety (“pseudophillipsite”) containing rather less silica has the formula (K2, Ca)2Al4Si5O18·9H2O. Crystals are monoclinic, but only complex cruciform twins are known, these being exactly like twins of harmotome (q.v.). 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 hardness is 412, and the specific gravity 2·2. The species was established by A. Lévy 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 basic volcanic rocks: e.g. in the basalt of the Giant's 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 Plombières and Bourbonne-les-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. (L. J. S.)