Simultaneous measurements of chlorite and illite crystallinity; a more reliable tool for monitoring low- to very low grade metamorphisms in metapelites; a case study from the Southern Alps (NE Italy)

Illite and chlorite crystallinity indices (XRD peak widths - IC and ChC) were determined for a suite of 238 samples derived from eight localities in the Palaeozoic pelitic-silty complex of the S Alpine basement of the Eastern Alps. Metamorphic grade in this region (related to the high-thermal-gradient Variscan metamorphism) ranges from the diagenetic zone in the E, through the anchizone up to the epizone in the W. Within the eight sample groups good agreement between the IC- and ChC-based classification of the specific low grade zones was found for five of the groups; this extends to all eight groups if only the ChC (001) data at low goniometer speed are considered, depending on the better correlation between ChC (001) and IC (002). Chlorite crystallinity can be considered as a useful complementary method especially where the interpretation of illite crystallinity is hampered by the presence of mixed-layer paragonite phases, margarite, etc.