Mineralogical properties of two profiles of volcanic ash soil from Costa Rica

Abstract

The mineralogical properties of two volcanic ash-derived stele, Binisito series, from the Irola and Victoria farms in Costa Rica were studied. Sand and silt fractions of field-wet soil were obtained by sieving the soil with water without removing organic matter or using dispersing agents. The sieved fractions were studied under a petrographic microscope, the finest fraction passing the finest mesh (37 µm), by DT A in a nitrogen atmosphere, and by X-ray fluorescence analysis. In addition to primary minerals, alteration products such as amorphous and secondary minerals were identified and described in detail. The mineral composition of the sifted fractions as well as the DTA of the finer fractions indicated that in the Irola profile, not only the B horizons but also the Ap and A horizons do not come from the same parent material. There is also a difference in parent material between the Irola and Victoria profiles. In the latter, hornblende is present, which is missing in the Hola samples. The fine fractions of the Irola profile consist mainly of allophane, while only the A12 profile contains an appreciable amount of gibbsite. The Victoria profile also contains allophane, but gibbsite is an important component, and its content increases with depth.

Fractionation with allophane alone without any pretreatment appears to be a necessary prerequisite for obtaining a true picture of the mineralogy of volcanic ash samples. The results indicate that valuable information can be obtained with this method, which would otherwise be lost if the usual pretreatments with H₂O and HCl had been used.