Abstract: Dredge haul MP43A, from a depth of 6180–10,860 ft on Sylvania Guyot, the companion seamount to Bikini Island, yielded large blocks of yellow buff-colored rock consisting of clay, opal, phillipsite, and veins of phosphatized foraminifera of basal Eocene age, suggesting more than 60 million years' exposure to sea water. These freshly broken blocks were partially covered with crusts of hydrated manganese and iron oxides. Occasional incompletely altered fragments indicate that the original volcanic rock was basaltic glass in a tuff breccia. The alteration products consist of: (1) water-clear phillipsite, (2) opal spherules which appear to have formed as linings of vesicles in the original glass, (3) a buff-colored, disordered, chloritie, mixed layer clay consisting of montmorillonite with interlayer ferric hydroxide.
The typical basaltic glass, weathered in the oceanic environment for much shorter periods of time, is converted to montmorillonite and phillipsite. The principal effect on the phillipsite of long exposure to sea water is etching of the crystal surfaces indicating non-equilibrium with sea water. The effect of the environment on montmorillonite is iron chloritization. Addition of iron occurs on a layer-by-layer basis, suggesting that hydroxy-iron polymers occur in sea water. Iron chloritization reduces the CEC of the MP43A montmorillonite by approximately one-third.