(Na, K)-phillipsite: Its stability conditions and geochemical role in the deep sea

Abstract

Phillipsite occurs in the deep sea in areas of very slow sedimentation and as the final alteration product of basalt, suggesting that it is thermodynamically stable. The thermodynamic data for gibbsite, kaolinite, K-feldspar, Na-feldspar, analcime and H 4 SiO 4 (aq), as well as the activities of seawater constituents are reasonably well known. These values and estimated entropies for Na- and K-phillipsites permit an estimate of the free energy of formation (at 25°C) for Na-phillipsite (5337.6 ± 6.7 kcal/mole) and for K-phillipsite (5382.4 ± 1.9 kcal/mole). The decomposition of phillipsite to analcime at 250°C yields the same result, suggesting that phillipsite is indeed a stable mineral. Phillipsite does not regulate the concentration of Na + and K + in seawater, but probably affects the pH of bottom waters and the composition of interstitial waters. No indications exist of “frozen in” equilibria from hydrothermal reactions at 100–200°C as could be expected by submarine volcanism.