SENEGALITA, AI2 (OH)3(H2O)(P04) , NOS FOSFATOS LATERÍTICOS DO PARÁ E MARANHÃO (AMAZÔNIA ORIENTAL)

MARCONDES LIMA DA COSTA, MARIA DE FÁTIMA FERNANDES REYMÃO

Resumo


Senegalite, Al2 (OH)3 (H20) (P04), from the occurrences of Pará and Maranhão (Eastem Amazônia), contrary that from Senegal, where ít ís found as a rare mineral, composes a very well-defined horizon up to 6 m thick in Cansa Perna (PA) and occurs as druses in Pirocaua (MA). Senegalite is orthorhombic, P21nb; a0 =7,658 ±1, b0 =9,690 ±2 and C0=7,595 ± 1 Å; Z=4, Ocalc. = 2,568 and H = 5,5 - 6,5. The strongest lines in the powder diffraction pattern are: 3,829 (l0), 4,10 (9), 5,39 (3) and 2,988 (8) Å,. The DTA curve shows three endothermic reactions, which correspond to the sucessive losses of crystallization and constitution water and to an exothermic reaction at 980ºC with ber1inite formatíon. The infrared spectrum points out the presence of OH and H20 groups as well as indicates symmetry reduction of the P04 tetrahedra. Senegalite frorn these occurrences was formed by íntense lateritization processes on phosphorous-rich rocks, and it occurs associated with variscite, crandallite-goyazite, wavellite, augelite, wardite, anatase and goethite/hematite, which is the typical mineral association of the laterites from the investigated region.

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