MICROSTRUCTURES OF INTERGROWN PHYLLOSILICATE GRAINS FROM VERRUCANO METASEDIMENTS (NORTHERN APENNINES, ITALY)

Interleaved phyllosilicate grains (IPG) of various compositions are wi despread in low-grade Verrucano metasediments of the northern Apennine s (Italy). They are ellipsoidal or barrel shaped, up to 300-400 mu m l ong and they are often kinked and folded; phyllosilicate packets occur as continuous lamellae or as wedge-shaped layers terminating inside t he grain. Using electron microscopy techniques (SEM, TEM) six types of IPG have been distinguished on the basis of their mineralogical compo sition: (1) Chl + Ms +/- Kln; (2) Chl + Ms + Pg +/- Kln; (3) Ms + Prl +/- Pg; (4) Ms + Prl + Su; (5) Ms + Prl + Chl + Su; (6) Su + Ms. Types (1) and (2) are mainly composed of chlorite, with Ms and Pg as minor phases; Kln grows on Ms in highly weathered samples. Types (3), (4), ( 5), and (6) are composed of muscovite, with intergrown Prl, Chl, Su an d new-formed muscovite. The IPG show all kinds of contacts: from coher ent grain boundaries with parallel basal planes and along-layer transi tions to low- and high-angle grain boundaries. The IPG formed on prist ine minerals such as chlorite and muscovite. The transformations took place during the prograde and retrograde metamorphic path of the rocks : they were facilitated by deformation and they occurred in equilibriu m with a fluid phase, which allowed cation diffusion. Prograde reactio ns [Chl = Ms (or Pg); Ms = Prl; Ms = Chl] involve dehydration and some times a decrease in volume, whereas retrograde reactions (Ms = Kln, Ms = Su) involve hydration and an increase in volume. These transformati ons do not simply occur through an interchange of cations, but often i nvolve deep structural changes: transitions from one phyllosilicate to another generally proceed through dissolution-recrystallization react ions. In conclusion, Verrucano IPG represent microstructural sites whi ch have not completely equilibrated with the whole rock and whose mine ral assemblage depends on the original composition of the microstructu ral sites.