A deep bluish green mineral, filling the numerous vesicles of a basalt, was found near Foz do Iguaçu (Parana State). This mineral was studied by means of petrographic microscopy, X-ray diffraction analysis, infrared absorption spectroscopy , electronic microscopy, differential thermal analysis and chemical analysis. The results of the investigations reveal that this mineral is celadonite. The studies of alteration of the Foz do Iguaçu basalt permit to suppose that the celadonite was a product of the activity of a hydrothermal solution or of a hot spring water .A deep bluish green mineral, filling the numerous vesicles of a basalt, was found near Foz do Iguaçu (Parana State). This mineral was studied by means of petrographic microscopy, X-ray diffraction analysis, infrared absorption spectroscopy , electronic microscopy, differential thermal analysis and chemical analysis. The results of the investigations reveal that this mineral is celadonite. The studies of alteration of the Foz do Iguaçu basalt permit to suppose that the celadonite was a product of the activity of a hydrothermal solution or of a hot spring water.Porto da Foz do Iguaçu (Estado do Parana) encontrou-se um mineral verde-azulado escuro enchendo numerosas vesículas de um basalto. Estudou-se esse mineral por meio de microscópio petrográfico, difração de raios X, espectroscopia de raios infra vermelhos, microscópio eletrônico, análise termo-diferencial e análise química. Os resultados das investigações mostram que esse mineral é celadonita. Os estudos da alteração do basalto de Foz do Iguaçu permitem supor que a celadonita foi produzida por solução hidrotermal ou por um manancial quente.