The crystal structures of nine trioctahedral mica-1M crystals (phlogopite and ferroan phlogopite containing Fe 3+ in tetrahedral position, ferroan tetra-ferriphlogopite, and titanian phlogopite), from ultramafic alkaline-silicate lavas and alkaline-silicate, silicate-carbonatite and carbonatite plutonic rocks occurring in the Alto Paranaíba Igneous Province (southeastern Brazil), were refined from single-crystal X-ray data in the space group C2/m to R values between 0.025 and 0.039. These mica crystals are characterized by low to very low Al content; therefore, Fe 3+ may be essential to fill the tetrahedral site. The octahedral position of all the crystals studied is characterized by variable amounts of Fe 2+ , Fe 3+ and Ti 4+ in substitution for Mg. The two independent octahedral sites, M1 and M2, show equivalent mean electron-density and mean bond-distances in most cases. Order at the octahedral sites is enhanced in the Ti-rich crystal from Presidente Olegario owing to the preference of Ti for the M2 site. The lateral parameters a and b reflect variations in tetrahedron composition, whereas the periodicity along c depends on octahedron composition. The crystal-chemical features of these micas are controlled by the peculiar composition of the rocks, by the high f(O 2), a(H 2 O), and f(CO 2), and by metasomatic events that have affected the Alto Paranaíba Igneous Province.
Abstract. After the nuclear weapons testing in the 1950s and 1960s,
the atmospheric tritium levels have almost returned to natural levels
(pre-1950 levels), which means that tritium is becoming more effective in
determining transit times in hydrological systems. It has also been
demonstrated that tritium is a non-reactive noble gas and in water is not
subject to chemical reactions, absorption, or dissolution/precipitation
processes, so it is conservative of the geochemical fingerprint of the
source. In addition, it is used as an effective tracer of water
contamination by landfill leachate, allowing to detect mixing percentages of
the leachate up to levels not achievable with normal chemical analyses (less
than 1 %). The purpose of the present work is to deepen the knowledge of
the hydrogeochemical characteristics of the aquifers in the pedemontana area
of the Venetian plain, and to formulate a conceptual framework of
underground water circulation, aimed at understanding phenomena of
contamination by toxic metals, harmful to human health. Tritium
concentration analyses were performed on selected samples of groundwater
collected in different wells in the south part of Treviso city to correlate
the young age of the water recharge. The abundance of young waters, in fact,
indicates a vulnerability of the aquifer to climate change with respect to
possible contributions of surface waters, in particular to prolonged drought
periods which could induce salinization dynamics.
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