Exotic sulphate and phosphate speleothems in caves from eastern Amazonia (Carajás, Brazil): Crystallographic and chemical insights

Abstract

This study investigates the mineral, crystallographic and chemical characteristics of sulfate and phosphate speleothems and the physicochemical conditions that enabled their development in caves in the banded iron formations and lateritic duricrusts of the Carajás region, in eastern Amazonia, Brazil. The X-ray powder diffraction (XRD), Rietveld refinement, scanning electron microscopy (SEM), electron microprobe (EMPA), thermogravimetry, as well as Raman, infrared, and Mössbauer spectroscopy techniques were used in this investigation. The chemical composition analyses of the guano and cave dripping water were performed by atomic absorption spectroscopy (AAS), spectrophotometry and multi-parameter probe. The results of the analyses revealed aluminite, felsőbányaite, jarosite, spheniscidite, phosphosiderite and amorphous phosphatic material in coralloid type speleothems while spheniscidite, phosphosiderite and strengite were identified in stalactite type speleothems. These minerals are the product of the direct chemical reaction of the bat guano, urine and bones accumulation (with K+, Na+, Ca2+, Mg2+, NH4+) and the iron and aluminum supplied by the lateritic duricrusts from the walls, ceilings and floor of the caves, into a mixture of sulfates and phosphates.