Corrensite and chlorite-rich Chl-S mixed layers in sandstones from the 'Macigno' Formation (northwestern Tuscany, Italy)

Corrensite and chlorite-rich chlorite-smectitc (Chl-S) mixed-layers occurring in the Macigno Formation of northwestern Tuscany were investigated by XRPD/SEM techniques. The samples from the Macigno outcrops were affected by metamorphism corresponding to the high-diagenesis/low-anchizone transition boundary. The basal spacing of corrensite measured from 00/reflections on EG-Mg2+-saturated samples ranges from 30.97 to 31.18 angstrom with a coefficient of variation (CV) of 0.46-0.77%, consistent with the rational pattern of a 1:1 Chl-S periodic stacking sequence. X-ray patterns of heated K+-saturated specimens reveal the presence of two types of corrensite, probably with different interlayer charges on their respective smectite-like layers. The chlorite-rich Chl-S mixed layers coexisting with corrensite consist of R2 chlorite (0.70-0.75)/S and chlorite (0.90)/S with long-range ordering R >= 1. SEM observations show that corrensite and chlorite-rich Chl-S mixed-layers are closely mixed on a sub-micron scale. Some grains of corrensite and chlorite-rich Chl-S mixed-layers include zones (2-10 mu m) of chlorite composition, as shown by microprobe analyses.