Si-deficient foitite with [4]Al and [4]B from the ‘Ługi-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 'Ługi-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 B 2 O 3 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 in the tourmaline-bearing sandstone with Ca saturated pore brines rich in Cl -and (BO 3 ) 3-.