Saturday, 15 July 2006

Mineralogy, Chemistry and Origin of Phosphor of Ceramics Found in Dark Earth Soils of the Amazon Region.

Marcondes L. Costa1, Marciléia S. Carmo1, Dirse C. Kern2, and Edivan C. Oliveira3. (1) Federal Univ of Para, Rua Augusto Corrêa nº 1, Guamá, Belém, 66075-110, Brazil, (2) Museu Paraense Emílio Goeldi, Av.Perimetral, 1901, Belém, Brazil, (3) Belágua, Estrada do Bituba-Sítio Santo Antônio s/n, Benevides-PA, 68795000, Brazil

Fragments of archaeological ceramics are very frequent in archaeological sites with Dark Earth all over the Amazon area. The technical methods of geology, mineralogy and the geochemistry have been applied with success in the study of the ceramics of archaeological sites of this region (Costa et al. 1991; 1993; 2001;2004a,b). An important Amazon area with great density of sites with Dark Earth is Caxiuanã, where the present work was accomplished. The ceramic fragments concentrate mainly on the A-horizons of the soil profile. The sites are 0,5 ha large, and A-horizon 23 to 31 cm in thickness inside of Archaeological Dark Earth (ADE). The tempers found in the ceramic fragments are cauixi, cariapé, sands and crushed ceramics. Cauixi and cariapé are the main tempers and they are formed of organic silica-rich substances amorphous to the x-ray diffraction. The fragments are constituted of quartz, clay-minerals partially crystalline (modified during burning) (illite and kaolinite), feldspar (albite), anatase, variscite and iron oxides like maghemite and amphibole some. The chemical composition shows little variation between the distinct fragments and the tempers, and it is independent of the horizon of the soil. The ceramic fragments are dominantly constituted by SiO2 (71.35%), besides of Al2O3 (8.6%), Fe2O3 (4.54%), P2O5 (1.31%), Na2O (0.98%), K2O (0.59%), TiO2 (0.4%), MgO (0.26%), CaO (0.19%). The LOI is relatively high (11.63%). The high values of SiO2 detach the high contents of quartz, besides cauixi and cariapé, and when associated to Al2O3 the clay minerals (burned kaolinite dominantly) and the feldspars. The still relatively elevated values of Na2O and and K2O confirm the presence of albite and illite, respectively. The P2O5 contents are relatively high and they meet in the form of amorphous phosphates of Al-Fe. Those values indicate contamination with phosphorus, coming from animal diet, and they show that the fragments can represent pottery of daily use. Cooking liberates phosphorous from the food into the liquid during the concoction, which reacts with the amorphous to cryptocristallines clay mineral of the ceramic wall leaving a form of amorphous aluminum phosphates (Costa et al. 1991). A contamination through soils is discarded, because the variations of P2O5 in the ceramic fragments do not display any dependence nor to the soils horizons, neither to the tempers. The raw material for elaboration of ceramic vessel of Caxiuanã was constituted of clayish material with kaolinite and quartz, and some illite. To them was added albite, rocks fragments (granites: quartz and feldspars), or cariapé and/or cauixi, separately, or even two or more of these tempers, which come from the surrounding area.

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