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\bibitem{Duarte_etal1979} {\textquotedblleft}\textit{Aspectos geoquimicos da Forma{\c c}ao Longa (Devoniano Superior do Piaui-Maranhao)}{\textquotedblright}. Duarte PJ, Mabesoone JM, Van Grieken R, Delgado A, Estudos geologicos \textbf{3}, 79 (1979)

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