Titanian clinohumite and geikielite in marbles from the Bergell contact aureole

Abstract

Abstract. The assemblage titanian clinohumite + forsterite + spinet + calcite is widespread in marbles from the eastern Bergell contact aureole (Switzerland/Italy). The Bergell titanian clinohumites vary considerably in composition (Ti0 2 : 0.19 to 2.05 wt%, F: 2.2 to 3.4 wt%). Electron microprobe analyses show that the titanian clinohumites contam less than detectable amounts of Co, Cu, Ni, Zn, Al, Cr and CI. No trace of ferric iron could be detected by Moessbauer spectroscopy. Moreover, the Moessbauer spectra indicate that Fe 2 + 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 TiO 2 M 1 (OH,F) 2 takes place at constant x 0 , 1 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 Cr 2 0 3 , Fe 2 0 3 and MnO, and thus different from those found in carbonatites, kimberlites and serpentinized ultramafic rocks.

Petrological and Geochemical Constraints on the Protoliths of Serpentine‐Magnetite Ores in the Zhaoanzhuang Iron Deposit, Southern North China Craton

“…We believe these minerals are not magmatic origin, but should be metamorphic genesis for the following evidences: (1) the iron orebodies are hosted within high-grade metamorphic country rocks, such as plagioclase-amphibolite, biotite-plagioclase gneiss and diopside-hornblende-plagioclase gneiss and their bedding orientation is parallel; (2) the 120° triangle equilibrated structure between the olivine and amphibolite (Figs. The titanian clinohumite in this study also carries F (~2.4wt%) similar to that in the marbles from Bergell contact aureole (Gieré, 1987), indicating its metamorphic nature. According to literatures, it was often paragenetic with forsterite, tremolite, spinel, chlorite, phlogopite, dolomite and calcite in high-grade carbonate rock under a small partial pressure of HF acid (Gieré, 1987;Bucher and Grapes, 2011).…”

“…Humite in the iron ores has major oxides of SiO 2 (37.59 -37.86wt % ), , FeO (6.99-7.56wt%), TiO 2 (1.28-1.94wt%) and F (2.31-2.42wt%), belonging to titanian clinohumite (Gieré, 1987). The concentration of all the other major elements are less than 0.5wt%.…”

“…Ilmenite is the predominant Fe-Ti oxide in the iron ores (ZK2702-2), with up to 20.72wt% MgO, FeO dominanted by geikielite end member (Gieré, 1987) with Mg # ~71. The magnesian ilmenite is free from Cr 2 O 3 and contains small amounts of ).…”

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain)

“…() for the host Chl‐harzburgites, showing high fluorine contents (Figure c). High‐ T metamorphic Ti‐clinohumite in sedimentary marbles—as, for example, from the western Betic Cordillera, Spain (Mazzoli et al., ) or from the Bergell contact aureole (Gieré, )—tend to have even higher fluorine contents, which often exceed the 1:1 TiO 2 Mg −1 F −2 exchange that is limited by the maximum Ti‐content in TiCl of X Ti = 0.5 (Figure c).…”

“…Ti-clinohumite in Milagrosa Cpx-serpentinite and Atg-Di-Dol rock has intermediate compositions between the two extremes, with X Ti = 0.28-0.39 and X F = 0.07-0.10 in cores and rim compositions of X Ti = 0.2 and X F = 0.18 (Figure 9c (Figure 9c). High-T metamorphic Ti-clinohumite in sedimentary marbles-as, for example, from the western Betic Cordillera, Spain (Mazzoli et al, 2013 ) or from the Bergell contact aureole (Gieré, 1987)-tend to have even higher fluorine contents, which often exceed the 1:1 TiO 2 Mg −1 F −2 exchange that is limited by the maximum Ti-content in TiCl of X Ti = 0.5 (Figure 9c).…”