SI-29 MAGIC-ANGLE-SPINNING NMR-SPECTROSCOPY QUANTITATIVELY MONITORS THE DOUBLE-CHAIN TRIPLE-CHAIN INTERGROWTHS IN HYDROUS SILICATES

Stacking phenomena play a central role in the kinetics of many mineral transformations, and quantitative determination of stacking sequences is therefore of major importance. The application of equilibrium thermodynamics to petrological problems depends on the correct interpretation of the thermodynamic status of the various regular and disordered stacking variants. Determination of the proportions of different polysome sequences (e.g. the ratio of double-chain to triple-chain silicates in an intergrowth) is essential to any thermodynamic study of polytypic/polysomatic transformations and disequilibrium time-temperature-transformation studies, from which the cooling histories of metamorphic rocks can be deduced. Although crystal intergrowths can often be imaged by high-resolution transmission electron microscopy, the technique is not suitable for the quantitative determination of polysome proportions. We have examined a polysomatic intergrowth of sodium clinojimthompsonite (hydrous triple-chain structure) and a chemically equivalent sodic amphibole (hydrous double-chain structure) and report that Si-29 magic-angle-spinning (MAS) NMR spectroscopy enables the populations of double- and triple-chain silicates in fine-scale lamellar intergrowths of the two polymorphs to be quantitatively determined.