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FOSFATOS DO TIPO CRANDALITA (PLUMBOGUMITA, GOYAZITA, GORCEIXCITA) RESULTANTES DO INTEMPERISMO DE AMBLIGONITA DE PEGMATITOS DE CORONEL MURTA (NORDESTE DE MINAS GERAIS) E SEU SIGNIFICADO PALEOCLIMÁTICO

1987; Sociedade Brasileira de Geologia; Volume: 17; Issue: 1 Linguagem: Inglês

10.25249/0375-7536.19874252

2177-4382

José Marques Correia-Neves, Vitória Régia Péres da Rocha Oliveiros Marciano, Jorge Carvalho de Lena, Antônio Carlos Pedrosa‐Soares,

Geography and Environmental Studies

This paper deals with crandallite type phosphates (plumbogummite, goyazite, gorceixcite) originating from amblygonite under weathering conditions active in very recent times in the Coronel Murta area (Northeastern Minas Gerais). Amblygonite, the crystallization of which took place about 500 Ma age within the replacement bodies of pegmatites emplaced in mica-bearing quartzites from the Proterozoic Salinas Group, was the starting material for the above mentioned supergene minerals. The pegmatitic veins, emplaced in the quartzites according to two perpendicular joint systems, underwent a strong weathering which produced the quasi total kaolinization of the pegmatitic primary feldspars observed at the present time. During the supergene processes, the amblygonite, after acting as a geochemical fence for Ca, Sr, Ba, Pb etc., which provided conditions for the formation of the crandallitic minerals, was itself transformed into kaolinite. The stability fields of these crandallitic minerals, compared to those of kaolinit e and amblygonite, show that they are easily formed under rather high pH values. As the environment becomes more acid and keeping in mind the very low cationic activities in groundwaters, all these phosphates become unstable and, under SiO] metasomatism, evclve to kaolinite . This is thermodynamical1y sound as revealed not only by the calculated stability diagrams but by the identified mineral assemblages as well. These mineral assemblages and their widespread regional scattering seem to be induced not only by climatic and relief conditions but also by their position within the weathering proflle. In the Coronel Murta area the most effective factors seem to have been the very recent climatic and relief changes.