ALLOPHANE, IMOGOLITE, AND GIBBSITE IN COATINGS IN A COSTA-RICAN ANDISOL

Neoformation of allophane, imogolite, and gibbsite has been studied ex tensively in soils in humid climates, but little is known about their micro distribution patterns. Such information may provide insight abou t the conditions at the micro-site scale, responsible for their format ion and disappearance. Our study describes amorphous and crystalline c oatings in a Melanudand on a 18,000 yr old andesitic lava in Costa Ric a. Fine-textured, white to yellow coatings could be observed in the B, C, and R horizon in the field. In thin sections the coatings are tran slucent and isotropic in plane polarized light (PPL), indicating that they consist of amorphous material. In the B horizon, under crossed po larized light, isotropic coatings show a gradual transition towards cr ystalline outer margins and spots, while in PPL such transition cannot be observed, suggesting a genetic relationship. Submicroscopical anal yses reveal that the amorphous coatings in the R and C horizon consist of allophanic material with a molar Al/Si ratio of 0.9, those in the B horizon consist of allophanic material and imogolite with a molar Al /Si ratio of 1.4. The amorphous coatings resulted from precipitation o f Al and Si liberated upon weathering of primary minerals in an initia l stage of lava weathering. The crystalline coating parts in the B hor izon consist of gibbsite and represent the ultimate stage of mineral n eoformation. The different coating composition in the soil horizons is the result of different leaching conditions at a macro and micro scal e. Care should be taken to compare results obtained at different sampl ing scales with different analytical methods.