Abstract
Mineralogical investigation of modern sediments off the coast of Peru and Chile reveals the presence of volcanic glass in many samples. The glass is commonly devitrified in sea water to fine aggregates of minerals, including calcite and kaolinite. The textural relations of these two phases suggest attainment of chemical equilibrium, and thus a thermo-dynamic analysis of their relationship is desirable. A 7-component system, including the phases dolomite, calcite, chlorite, kaolinite, and quartz, is chosen for study. This system may be analyzed graphically if quartz is present in excess and H 2 O and CO 2 are taken as "mobile" components. The expected phase associations under a given set of conditions are studied in detail and agree with existing petrographic data; the agreement shows that the hypothesis of chemical equilibrium among these phases in sedimentary rocks should be seriously entertained. The idea of incompatible relationship between calcite and kaolinite is reviewed and the evidences for it are not compelling. Possible complications due to other clay minerals and other components are briefly considered. The most likely sources of trouble are montmorillonite and the mixed-layer clays. Depending on its composition, the montmorillonite may change the geometry of the phase diagrams but will not affect the calcite-kaolinite binary join. Whether the mixed-layer clays will change the diagrams depends on whether such clays behave as mono- or polyphase systems, about which nothing is as yet known.
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