Phillipsite is a mineral series of the zeolite group; a hydrated potassium, calcium and aluminium silicate, approximating to (Ca,Na2,K2)3Al6Si10O32·12H2O.

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 Bourbonne-les-Bains, in France.

Minute spherical aggregates embedded in pelagic red clay were dredged by the Challenger from deep sea sedimentary deposits in the Pacific Ocean.

[3] It has been discovered that the volcanic ash that Romans employed in the mix for construction of harbor piers and sea walls contained phillipsite, and that an interaction with sea water actually causes crystalline aluminous tobermorite structures in the mortar to expand and strengthen, making the material substantially more durable than modern concrete.