Studies on the weathering and alteration of minerals by their fine texture analysis

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In the Monbusho International Scientific Research Program since 1978 we sampled many rocks and minerals at various localities in the carbonatitealkaline complexes %, ; hich dot Brazilian shield. Main subjects of the present project are the surveys of weathering and alteration of the minerals with optical and electron microscopy, EPNIA and/or X-ray diffraction. The principal results are summarized as follows, in addition to the analysis of the general features of weathering and alteration.(1)Anatase crystals with a peculiar morphology have been mined together with phosphates for fertilizer in Tapira, Minas Gerais. The present studies on the fine texture and chemical composition of this anatase and perovskite obtained from the same mine, indicate that the anatase should be a weathering and alteration product of perovskite, and its peculiar morphology could be explained as a pseudomorph of perovskite.(2)The occurrence and exsolxition textxire of Ti-bearing magnetite collected in carbonati … More te-alkaline complexes have been investigated in order to help otir study on the origin of carbonatite. The present investigation has demonstrated that the magnetite often exsolves geikielite(MgTiO_3), irrespective of its age of intrusion or locality in Brazil.(3)On the investigation into a mica ivith unusual zoning, which obtained from carbonatites, we have reached the conclusion that it is closely related to the "alkaline metasomatism" caused by action of carbonate magma.(4)The occurrence of polymignyte, zirkelite and zirconolite in glimmerite, all of which belong to the Ca-Zr-Ti-O system, has been described.(5)Fine texture which arises from the contact metasomatism between jacupirangite and carbonate magma has been electronmicroscopically studied.(6)Lateritic weathering soils from carbonate rocks have been chemically investigated with radio activation analysis.(7)We report geochemical and isotopic data of carbonatite specimens from five Brazilian localities. These analyses indicate that carbonatite melts attribute their origin to asthenospheric mantle plume. Less