PHOSPHATE MINERALS IN A LATERITIC CRUST FROM VENEZUELA

Ferruginous crusts and pisolites have chemical and mineralogical prope rties that differ from the surrounding soil due to Fe and Al enrichmen ts which cause cementations that can harden irreversibly. In addition to, and possibly as a result of the Fe and Al accumulation, other ions , particularly phosphate are often also enriched by a factor of 2-20 r elative to the surrounding soil. The P accumulated in ferruginous mate rials is normally bound to the Fe or Al in amorphous forms of low solu bility. Distinct minerals have rarely been identified. We examined a s ection through a Venezuelan ferruginous crust, which contained portion s with P contents >100 g kg(-1), chemically, mineralogically and micro morphologically with the aim to show some of the mechanisms that cause such extreme P accumulation and segregation in a landscape that is ot herwise very nutrient poor. Except for the cementation, manifested as an in-filling of pores by Fe, the morphology of the ferruginous crusts reflected that of the original soil. At approx. 30 cm below the crust 's surface, goethite, strengite and leucophosphite (KFe2(PO4)(2)OH . 2 H(2)O) were identified along a downward sequence of pores nearer the s urface to pores at greater depth to the matrix of the lower crust. Whi le the lower crust contained highly soluble P,Fe oxides from outer por e spaces showed high P sorption. The element and mineral distribution across thin sections suggested that incoming Fe had interacted with a soil matrix that was exceptionally rich in K and P to form highly solu ble leucophosphite, followed by less soluble strengite and finally Fe oxides that essentially occluded the more soluble minerals found in th e lower crust. Associated organic C dated at 18,700 y b.p., suggesting that the occlusion process occurred around the last glacial maximum, when the region became more arid. Although extreme in its extent, the process of separation and occlusion of minerals demonstrated here, may be useful for interpreting similar processes in soils and soil cement ations that affect the biogeochemical turnover of elements.