THE ROLE OF RANDOMLY MIXED-LAYERED CHLORITE SMECTITE IN THE TRANSFORMATION OF SMECTITE TO CHLORITE/

Vesicular and groundmass phyllosilicates in a hydrothermally altered b asalt from the Point Sal ophiolite, California, have been studied usin g transmission electron microscopy (TEM). Pore-filling phyllosilicates are texturally characterized as having coherent, relatively thick and defect-free crystals of chlorite (14 Angstrom) with occasional 24-Ang strom periodicities. Groundmass phyllosilicates are texturally charact erized as 1) randomly oriented crystals up to 200 Angstrom in width an d 2) larger, more coherent crystals up to 1000 Angstrom in width. Smal l crystallites contain predominantly 14-Angstrom layers with some 24-A ngstrom units. Large crystals show randomly interlayered chlorite/smec tite (C/S), with approximately 50% chlorite on average. Adjacent smect ite-like layers are not uncommon in the groundmass phyllosilicates. El ectron microprobe analyses show that Fe/Mg ratios of both groundmass a nd vesicular phyllosilicates are fairly constant. Termination of bruci te-like interlayers has been identified in some of the TEM images. The transformation mechanisms represented by these layer terminations are 1) growth of a brucite-like interlayer within smectite interlayer reg ions and 2) the dissolution and reprecipitation of elements to form ch lorite layers. Both mechanisms require an increase in volume as smecti te transforms to chlorite. The data, combined with that from previousl y published reports, suggest that randomly interlayered C/S is a metas table phase formed in microenvironments with low water/rock ratios. Ch lorite forms in microenvironments in the same sample dominated by high er water/rock ratios. The relatively constant number of Mg's in the st ructure (Mg# of both structures indicates that in both microenvironmen ts the bulk rock composition has influence over the composition of phy llosilicates.