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

A tourmaline depleted in Si has been found in the cement of Upper Rotliegend aeolian sandstone overlaying Lower Rotliegend volcanic rock in the ‘ ugi-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 μm 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 [4]Al and [4]B. Compositions of the tourmaline in successive analytical spots show that Si deficiency is supplemented both by [4]Al (0.00 to 0.48 Al apfu) as well as by [4]B (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 of the grain facies in the tourmaline-bearing sandstone with Ca saturated pore brines rich in Cl- and (BO3)3-.