Crystal-chemistry and thermal behavior of Fe-carpholite from the Pollino Massif (southern Italy)

The crystal chemistry and the thermal evolution of Fe-carpholite from the Pollino Massif have been investigated by means of a multimethodic approach. A combination of optical microscopy, Secondary Electron Microscopy (SEM) analyses, μRaman spectroscopy, thermal analysis (Differential Thermal Analysis, DTA; Thermogravimetry, TG), room temperature Single Crystal X-Ray Diffraction (SCXRD) and in situ High Temperature X-Ray Powder Diffraction (HT XRPD) was employed. Field and micromorphological observations showed that the studied carpholite occurs in veins embedded in fine grained matapelites and is associated to quartz, calcite, chlorite and phengite. Tiny carpholite crystals are closely associated with quartz suggesting simultaneous formation. Structure refinements from single crystal X-ray diffraction confirm that carpholite crystallizes in the Ccce space group. Anisotropic refinements converged at 2.3 ≤ R (%) ≤ 2.6 and provided unit cell parameters a ~ 13.77 Å, b ~ 20.16 Å and c ~ 5.11 Å and V ~ 1419 Å3. XFe (i.e. the molar fraction Fe2+/(Mg+Fe2++Mn)) ~ 0.6 was derived from the refined occupancy at the M1 site and is correlated to the lattice expansion mainly along the b and a axes and to geometrical distortions of the M1, M2 and M3 octahedra. μRaman spectrum of unoriented Fe-carpholite crystals evidenced a dense succession of bands in the 200-1200 cm-1 spectral region as well as a strong peak at 3630 cm-1 and a weak peak at 3593 cm-1 which account for the presence of two independent OH groups also provided by the structure refinement. The TG curve indicates a total mass loss of 15.6% in the range 30-1000°C whereas the DTA curve showed a broad endothermic band at ~ 400°C, extending up to ~ 650°C and weak exothermic peaks at ~700 and 750°C. The latter may be ascribed to the breakdown of the Fe-carpholite structure and crystallization of new phases. The in situ high temperature X-ray powder diffraction, allowed to follow the pattern evolution in the temperature range from 30 to 1105°C. Rietveld refinements were carried out for all patterns collected from 30 to 630°C. No significant modifications were observed from 30 to 355°C whereas reflection splitting appeared starting from 380°C as a consequence of a Fe-oxidation/deprotonation process. The carpholite and the deprotonated carpholite phases coexist in the 380-580°C temperature range whereas the only deprotonated phase is observed up to 630°C. Above this temperature the carpholite structure collapses and the characteristic peaks of spinel and quartz phases are observed. At 1105°C, spinel, mullite, garnet, cristobalite and tridymite may be clearly identified. The present work contributes to fill the gap on the thermal stability of Fe-carpholites and may have implications for a better understanding of the thermal evolution of HP/LT metasediments.