The evolution from disordered Ad to ordered 2M1 white K-mica polytype in low-temperature metamorphosed sedimentary rocks

Michael Dalla Torre, Martin Frey
Six sets of samples from five low-temperature regional metamorphic regions were investigated m order to determine the evolution of white K-mica polytypes as a function of temperature The data show that disordered group A structures (Ad) associated with small amounts of mixtures of 1M and 2Mj structures occur m the diagenetic zone. The true lMd structure was found to be less common than suggested by previous authors. At anchizonal grades, Ad and lMd structures disappear and mixtures with
more » ... amounts of 1M and 2M, polytype prevail In 90% of the samples the concentration of 2M! varies between 70 and 90% at this grade. The largest proportions of the 1M structure are 40% and were found m a sample belonging to the diagenesis/anchizone boundary. At the onset of the epizone, 100% 2M, are reached m most cases including a few exceptions Our data suggest that the polytype transformation from Ad structures at the diagenetic zone to a mixture of 1M and 2M, at the anchizone to 100% 2M, at the epizone is a function of temperature and can be used as an indicator of grade in sedimentary rocks metamorphosed at lowtemperature conditions Keywords white K-mica, lllite, polytype, X-ray diffraction, Central Alps,
doi:10.5169/seals-58477 fatcat:f4q4s2gopjfzdjwgu5lry6xytm