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

Abstract 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.