Si-deficient foitite with [4]Al and [4]B from the ‘Ługi-1’ borehole, southwestern Poland

Abstract

A tourmaline depleted in Si has been found in the cement of Upper Rotliegend aeolian sandstone overlaying Lower Rotliegend volcanic rock in the 'Lugi-1' prospecting borehole near Jarocin in the Fore-Sudetic Monocline, south-western Poland. Tourmaline, representing Mg-bearing foitite, crystallized around quartz grains in the form of radial aggregates of tiny crystals, reaching only 1-2 mu m in diameter. Due to the very small size of the tourmaline crystals and the presence of significant contents of transitional metals in the crystal lattice, only direct determination of B2O3 in nm-sized spots enables evaluation of Al-[4] and B-[4]. Compositions of the tourmaline in successive analytical spots show that Si deficiency is supplemented both by Al-[4] (0.00 to 0.48 Al apfu) as well as by B-[4] (0.00 to 0.83 B apfu) at varying proportions. The origin of the mineral was related to diagenesis of evaporate sediments inducing reactions of quartz and clay minerals as primary components in the tourmaline-bearing sandstone with Ca saturated pore brines rich in Cl- and (BO3)(3-).