Phillipsite of Saline Lakes and Soils

Phillipsite has formecl from volcanic glass in saline, alkaline lakes both saturated and undersaturated with respect to gaylussite and trona. It has also formed from volcanic glass in saiine, alkaline soils of Olduvai Gorge, Tanganyika. Phillipsite occurs both as lath,shaped and as equant crystals, which are commonly zoned lvith refractive indices ranging between 1.438 and 1.490. Molecular SiOz/RzOr ratios are 5 | lor analyzed phillipsite of trachytic origin and 6.0 to 6 7 for rhyolitic phillipsite. All analyzed phillipsites are high in aikali ions and very low in aikaiine-earths ions. Phillipsite occurs in seemingly compatible association with authigenic analcime, searlesite, K-feldspar, clinoptilolite, erionite, chabazite, natrolite, and dan-sonite. Phillipsite can form in a few hundred years, and a few thousand years are sufficient for complete zeolitic alteration of tufi beds. Optimum conditions for the rapid formation of phillipsite from volcanic glass appear to be a pH above 9 and a Na/K ratio between 10 and 50. Hydration and a large loss of silica are involved in forming phillipsite from rhyolitic glass. Phillipsite similar to that of saline lakes can be synthesized in the laboratory under conditions rather ciosely paralleling the saline lake environment. Dominance of phillipsite over clinoptilolite in saline, alkaline lakes may reflect a lorv activity of silica, and clinoptilolite appears to be favored over phillipsite r.here the activity of silica is high. Absence of heulandite in deposits of saline, alkaline lakes may' reflect the very low activity of Ca2+ in this environment