A Qualitative Mineral Analysis of the Coarse Fraction of an Alonso Soil

Abstract

Quartz, hematite, magnetite and maghemite were identified as the most prominent minerals present in the coarse fraction of a typical Alonso clay. Although plagioclase was an important constituent detected in the parent rock, only traces were found in the soil. There was no indication of the presence of orthoclase either in the rock or in the soil. The iron minerals appear to follow a rather uniform distribution throughout the profile, while the most intensive peaks of quartz were more prominent in the silt rather than in the sand fractions. A possible explanation for the particle-size distribution of quartz could be the original textural composition of the pedogenic material as would be encountered in quartziferous volcanic ashes. Inasmuch as the percent of silt in the soil decreased with the profile depth, it is reasonable to expect a concomitant decrease in the relative quartz content of the soil.