Evidences of talc-white mica assemblage in low-grade metamorphic rocks from the internal zone of the Rif Cordillera (N Morocco)

Abstract Talc-phengite intergrowths were identified in epizone rocks from the Beni Mezala unit (Internal Zone of the Rif). These rocks show the non-common assemblage quartz-K-feldspar-albite-phengite-talc-vermiculite-chlorite/vermiculite mixed-layers. Although X-ray diffraction patterns indicate the presence of talc, talc could not be observed petrographically. In contrast, transmission electron microscopy indicates that white mica consists of nanometer-sized intergrowths of phengite and talc. At the scanning electron microscope scale, these micas show anomalously high contents of Fe + Mg composition (with an average value of 0.51 atoms per formula unit, apfu). The subtraction of the talc component from the mica formula allowed to obtain a mica with a limited phengitic substitution K0.91Na0.03(Al1.74 Fe2+0.23Mg0.03Ti0.03)Σ=2.03(Si3.18Al0.82) which permits an estimation of minimum pressure (P)