Bauxite deposits in the Amazon region are commonly covered by yellowish or reddish clays that can reach up to 25m thick, known as Belterra Clay (BC), term attributed after Sombroek in 1966 describe similar clays, but not related to bauxites in the region of Belterra, Lower Amazon. In Rondon do Pará, the BC has between 10 and 13 m thick, covering world-class bauxite reserves. Representative samples of BC from three pilot bauxite mines (Branco, Décio and Ciríaco) in Rondon do Pará were studied. The BC covers laterite-bauxite regoliths with well-defined horizons. From the basis to the top, they are composed of: bauxite clay, followed
by massive bauxite, massive iron-aluminous crust, dismantled iron-aluminous crust,
ferruginous spherolites, and a horizon with bauxite nodules in a clayey matrix. In discordant contact with the lateritic profile, the BC has reddish brown colors at the base, with yellowish tones towards the top. It has a homogeneous appearance with no visible sedimentary structures, silt-clayey aspect and with fragments of porcelain bauxite (popcorn like) at its base. The mineral composition of the material is dominated by kaolinite, with Al-goethite, gibbsite, hematite, anatase and residual quartz. The SiO2/Al2O3 ratio of about 1.050 is lower than the theoretical kaolinite (1.178), confirming the existence of gibbsite and even Algoethite as aluminum carrier. As observed by X-ray diffraction analysis, kaolinite is of low structural order, evidenced by the low distinction of the diffraction bands at the 19-22° 2ϴ
(Cu anode) region and because this mineral has no satisfactory correlation with any structural model of kaolinite, which was considered the principal difficulty in the application of the Rietveld method. Al-goethite has about 32 mol% of Al and is responsible for the ocher / yellowish color of the BC. This mineral associated with hematite and amorphous iron hydroxide (up to 0.5% Fe) is responsible for up to 13% of Fe2O3 contents. Quartz rarely exceeds 1% as fragments in the clayey matrix. Anatase has concentrations between 2.4 and 2.8%. As observed by SEM, with the exception of quartz, the minerals represent pseudohexagonal crystals measuring from 150 to 700 nm. The thermal behavior of the material confirms its mineralogical composition. Its mineral quantification by thermogravimetric analyzes is in accordance with the Rietveld method and stoichiometric (FRX) results, despite
a precise correlation between these methods was unfeasible.
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