WHITE MICAS WITH MIXED INTERLAYER OCCUPANCY: A POSSIBLE CAUSE OF PITFALLS IN APPLYING ILLITE KüBLER INDEX ("CRYSTALLINITY" FOR THE DETERMINATION OF METAMORPHIC GRADE

Abstract

Integrated microstructural observations, X-ray powder diffractometric (XRPD) modal composition, illite Kubler index and chlorite “crystallinity” determinations and vitrinite reflectance measurements were carried out on marly slates from selected profiles of the Helvetic zone of the Central Alps, Switzerland. The studied profiles were: Upper Jurassic from the Wildhorn nappe, Brienz, Upper Jurassic from the Parautochthonous of the Aar massif, Glarus Alps, Eocene from the Griesstock nappe of the Glarus Alps and Upper Jurassic from the Axen nappe, Rhine Valley. In some of the localities studied, illite Kubler index (“crystallinity”) values were anomalously high, yielding only diagenetic conditions, while chlorite “crystallinity” and vitrinite reflectance showed anchi- and epizonal metamorphic conditions. Detailed XRPD observations carried out on Ca-saturated and glycolated mounts indicated subordinate amounts of swelling (smectitic) interstratifications in white mica. In addition to the dominant K-white mica, traces of discrete paragonite and paragonitic phases and tobelitic impurities in the form of either regular interstratifications or micas with mixed (K≫Na>NH4) interlayer compositions could be detected from the XRPD (00,10) basal reflections. On the basis of the organic maturity assumed from vitrinite reflectance, probable partitioning of N and H between organic and inorganic phases, and the results of elemental (C, H, N, S) analyses carried out on the <2 μm fraction decarbonated and oxidized samples, the amount of NH4+ fixed in inorganic phases could be estimated. Small, but systematically appearing, absorption bands between 1400 and 1440 cm−1 in the FTIR spectra unequivocally proved the presence of ammonium in the rocks studied. Small amounts of N within the mica flakes were detected by electron energy loss spectrometry (EELS), confirming that NH4+ is indeed fixed in the interlayer site position. Energy dispersive spectroscopy (EDS) using scanning transmission electron microscopy (STEM) revealed heterogeneities in the interlayer cation occupancies. Although K is always dominant, irregular, local, domain-like enrichments in Na could be seen. The most accurate model to describe the disequilibrium state of the incipient metamorphic white micas studied is that of a dioctahedral mica structure with irregularly varying interlayer occupancies combined with subordinate amounts of swelling mixed-layers. The present work shows that, if these white micas are to be used for metamorphic petrogenetic purposes, special attention should be paid to their detailed characterization, especially in organic matter rich lithologies often characterized by high Al/Si bulk chemical ratios. Seemingly, white micas with mixed interlayer occupancies may be more widespread than has generally been anticipated so far.