Fine-scale chlorite muscovite association in low-grade metapelites from Nurra (NW Sardinia) and the possible misidentification of metamorphic vermiculite

Chlorite-muscovite porphyroblasts occur in the low-grade metapelites from Nurra (NW Sardinia). Although actually heterogeneous and built up by two different intermixed minerals, the porphyroblasts strongly simulate optical homogeneity and produce microprobe analyses that are typical for metamorphic vermiculite. Transmission and scanning electron microscopy, combined with analytical facilities, leads to a deeper characterization of the porphyroblasts, that actually consist of a random association of chlorite and muscovite. As a further result, the present study also shows that the existing criteria used to identify metamorphic vermiculite, namely optical features and microprobe composition do not warrant the correct identification. Finally, the present results once more confirm the importance of back-scattered electron imaging in the SEM as a powerful tool for the routine petrographic study of sedimentary and low-grade metamorphic rocks.