Titanian clinohumite and geikielite in marbles from the Bergell contact aureole

The assemblage titanian clinohumite+forsterite +spinel+calcite is widespread in marbles from the eastern Bergell contact aureole (Switzerland/Italy). The Bergell titanian clinohumites vary considerably in composition (TiO₂: 0.19 to 2.05 wt%, F: 2.2 to 3.4 wt%). Electron microprobe analyses show that the titanian clinohumites contain less than detectable amounts of Co, Cu, Ni, Zn, Al, Cr and Cl. No trace of ferric iron could be detected by Moessbauer spectroscopy. Moreover, the Moessbauer spectra indicate that Fe²⁺ occurs only in one of the five octahedral positions in the crystal structure of the studied titanian clinohumite. Under the conditions of the contact metamorphism (600–650° C, 3 kb total pressure) the compositional variation along the exchange vector TiO₂M_–1 (OH,F)_–2 takes place at constantx_OH which is fixed by the pore fluid. Titanian clinohumite sometimes contains geikielite inclusions which strongly fractionate Fe and Mn relative to titanian clinohumite. The geikielites from the Bergell marbles are poor in Cr₂O₃, Fe₂O₃ and MnO, and thus different from those found in carbonatites, kimberlites and serpentinized ultramafic rocks.