Abstract: Phillipsite is a common constituent in diagenetically altered rhyolitic vitric tufts of Cenozoic saline, alkaline-lake deposits where it occurs as spherulites or aggregates of prismatic crystals. This phillipsite has chemical and physical properties that differ from phillipsites that occur in rocks of other compositions and depositional environments, but previously it had not been completely characterized. Published analyses of five samples from four deposits in the southwestern United States show that the phillipsite is very siliceous and alkalic; the Si/(Al + Fe3+ ratio is 3.08–3.37, alkalis greatly exceed the divalent exchangeable cations, and the Na/K ratio is 1.05–3.05. The mean index of refraction (1.451–1.470) is much lower than that generally reported for the phillipsite group and can be correlated with the relatively high Si and alkali contents. Monoclinic unit-cell parameters show the following ranges: a = 9.931–9.991 Å, b = 14.142–14.155 Å, c = 8.650–8.706 Å, β** = 124.57–125.07°, and V = 1000.3–1007.4 Å3. The especially small b dimension is consistent with the high Si content.