A peculiar textural association of muscovite, paragonite and chloritoid occurs in a micaschist bearing muscovite and garnet porphyroblasts, from the Greiner Unit (Eastern Alps, Italy). Three different tectonometamorphic stages can be recognised in this rock: 1) the early development of S1 preserved only as relic within garnet porphyroblasts; 2) a syn-kinematic crystallisation of chloritoid and muscovite along an early foliation S2; 3) the development of a crenulation cleavage (S3) associated with the growth of chlorite, muscovite and paragonite along S3 and porphyroblastic garnet and muscovite. Both garnet and muscovite porphyroblasts show inclusion trails of chloritoid, quartz and ores, which are parallel to S2. Furthermore, the muscovite porphyroblasts include paragonite crystals which are parallel to S2 and display a rectangular to elongate “blocky shape” in thin section. The elongation is nearly perpendicular to the muscovite (001) basal plane (but with the same cleavage traces as those of the muscovite).
An HRTEM/SAED/AEM study of the muscovite/paragonite single crystals reveals two different situations: 1) sharp boundaries for the “blocky shaped” paragonite crystal within muscovite porphyroblasts with the angle of the (001) basal plane across the boundary being that required for adjusting the different basal spacing; 2) the presence of K-rich paragonite micro-domains within Na-rich muscovite porphyroblasts; both are 2M1 polytypes and in structural continuity, with an angle between [00l]*paragonite and [00l]*muscovite that varies from 0 to 5° in order to accommodate the layer-thickness misfit. This results in a “tweed texture”, suggesting a further decomposition inside the muscovite-paragonite solvus.
This peculiar muscovite-paragonite textural association is a microstructural relic of a multistage metamorphism which affected these rocks with evidence of an isothermal increase of pressure due to the reaction: Fe-rich Cld + Narich Ms + Qtz → Phe-rich Ms + Pg + Chl ± Mg-rich Cld + H2O.