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 '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-.

Tourmaline at diagenetic to low-grade metamorphic conditions: Its petrologic applicability

“…For a series of 28 tourmaline analyses, the range of the structural formula values is: (Na 0. Pieczka et al (2011) suggest that the Si deficiency in the tetrahedral site is filled by B, consistent with crystallographic assignments (e.g. Ertl et al, 2008;Schreyer et al, 2002).…”

“…Panel d: data of Sperlich et al (1996). low-temperature tourmaline has been found in carbonate rocks, stromatolites, evaporites, conglomerates, sandstones and mudstones that have experienced diagenesis to low-grade metamorphic conditions and have also been found in relatively low temperature (b300°C) geothermal systems that interact with a variety of host rocks (e.g. Moore et al, 2004;Novoselova and Sosedko, 1991;Pieczka et al, 2011;Ricketts, 1978;Srivastava and Schnitzer, 1976;Van den Bleeken et al, 2007).…”

“…These tourmalines are typically small and euhedral, have pale coloration and can have a range of morphologies from very fine prismatic crystals to radial aggregates to equant grains Pieczka et al, 2011;Ricketts, 1978;Spencer, 1925). Such Fig.…”

“…However, an example of one of these occurrences is in the gas-bearing Permian Upper Rotliegend aeolian quartz and sublithic arenitic sandstone of southwestern Poland (Pieczka et al, 2011). These sedimentary rocks, recovered from a depth of 2660 m, overlie the Lower Rotliegand volcanic unit, underlie evaporitic rocks and are considered to have developed in an arid, aeolian setting with development of local evaporites.…”

“…6b). Pieczka et al (2011) argue that the tourmalines were developed by diagenetic processes associated with quartz and clay minerals reacting with Ca-saturated brines containing Cl 1− and (BO 3 ) 3− ions -constituents originally derived from ephemeral lake deposits developed in an arid environment. The relatively high amounts of X ❑ and moderate-to-low contents of Na and Ca of the X-site appear to reflect tourmaline formation in a fluid with this composition, and the relatively low Mg/(Mg + Fe) ratios are consistent with the coexistence of ankerite in the cements.…”