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

The crystal chemistry and thermal behavior of Fe-carpholite from the Pollino Massif have been investigated by a multi-method approach. A combination of optical microscopy, scanning electron microscopy, mRaman spectroscopy, thermal analysis, room-temperature single-crystal X-ray diffraction, and high-temperature X-ray powder diffraction was employed. Field and micromorphological observations showed that the studied carpholite occurs in veins embedded in fine-grained matapelites and coexist with quartz, calcite, chlorite, and phengite. In particular, the 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 yielded unit-cell parameters a ∼13.77 Å, b ∼20.16 Å, c ∼5.11 Å, and V ∼1419 Å3. An XFe [i.e., the molar fraction Fe2+/(Mg+Fe2++Mn)] of ∼0.6 was derived from the refined occupancy at the M 1 site and is correlated to structural expansion mainly along the b and a axes and to geometrical distortions of the M 1, M2, and M3 octahedra. mRaman spectrum of unoriented Fe-carpholite crystals exhibits several bands in the 200-1200 cm-1 region, a strong peak at 3630 cm-1 and a weak peak at 3593 cm-1, the latter two of which account for the presence of two independent OH groups, as also revealed by the X-ray structure refinement. The TG curve indicates a total mass loss of 15.6% in the temperature range 30-1000 °C, and the DTA curve shows 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 from 30 to 1105 °C revealed no significant changes in XRD patterns from 30 to 355 °C but reflection splittings from 380 °C due to a Fe-oxidation/deprotonation process. The carpholite and deprotonated carpholite phases coexist in the temperature range 380-580 °C, whereas only the 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 can be clearly identified. Our results provide insight into the thermal stability of Fe-carpholites and may help understand the thermal evolution of HP/LT metasediments.