Columbite-tantalite minerals from Val Vigezzo (VCO province, Piedmont, Italy): a preliminary study

Columbite-group minerals (CGMs) have general formula AB2O6 (usually A = Fe2+, Mn2+ and B = Nb5+, Ta5+) and are well-spread in pegmatitic settings. This group of minerals has petrogenetic significance, as their texture reflects variations occurred in the crystallization environment, i.e. during the magmatic and hydrothermal evolution of granitic/pegmatitic systems (e.g. Sarbajna et al., 2000) and are important from a crystallographical point of view as well. CGMs are found with ordered cation distributions, where the A site is exclusively occupied by Fe and Mn and the B site by Nb and Ta, as well as with disordered distributions, where no distinction between the A and B site is present and the structure is ascribable to that of ixiolite (e.g. Chukanov et al., 2023). Here, we report a preliminary study by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), both coupled with an energy dispersive x-ray spectroscopy (EDS) system, of a set of four crystals from the tertiary alpine pegmatites of Val Vigezzo (Verbania-Cusio-Ossola province, Piedmont, Italy): Piano dei Lavonchi (PLV1), Pizzo Marcio Nord (PMN2 and PMN6-2) and Rio Vasca (RVS). The crystals are found within different types of rocks, such as unaltered quartz-feldspar pegmatites, as well as heavily-altered, completely-albitized units. In BSE images, CGMs are characterized by different types of compositional zoning, namely patchy and growth zoning. Sample PLV1 has a growth zoning, with a columbite-(Fe) composition for both the core and the rim; PMN2 is an inversely-zoned crystal, with a tantalite-(Mn) core, a borderline columbite-tantalite-(Fe) intermediate rim and a columbite-(Fe) external rim; PMN6-2 is a normally-zoned columbite-(Mn) crystal, with a rim richer in Mn than the core; RVS is a patchy-zoned crystal, with tantalite-(Fe) portions mixed up with columbite-(Fe) portions. At the TEM scale, the studied thin region has a columbite-(Fe) composition in the case of PLV1; tantalite-(Fe) composition for PMN2; columbite-(Fe) for PMN6-2; tantalite-(Fe) composition for RVS. The results indicate a great variability in the crystallization conditions during pegmatitic cooling. Unit cell parameters, obtained from calibrated electron diffraction patterns (Capitani et al., 2006), taken along zone axis are: a = 14.373(5), b = 5.754(2), c = 5.067(2) Å for PLV1, a = 14.353(4), b = 5.627(2), c = 5.076(2) Å for PMN2, a = 14.401(6), b = 5.716(2), c = 5.080(2) Å for PMN6-2, a = 14.448(9), b = 5.809(8), c = 5.075(6) Å for RVS. A plot of these parameters in the a-c diagram (Ercit et al., 1995) shows that these columbite crystals fall in the ordered samples region.