Weathering of primary minerals in volcanic ash-derived soils of the central cordillera of Colombia

Abstract

Four soils derived from recent volcanic ash were studied along an altitudinal transect on the east slope of the central Cordillera of Colombia, South America. Average annual temperature ranged from 8°C to 24°C. Mineralogical studies showed that the sand and silt fractions of the four soils consist of a similar suite of minerals, some of which, particularly pyroxenes, hornblende, volcanic glass, and feldspars, decrease in amount with decreasing altitude and increasing temperature. The distribution and condition of minerals within and between soils suggests a stability sequence for these soils as follows: quartz ⩾ cristobalite ⩾ magnetite, vermiculite, sodic amphibole > plagioclase(albite/andesine) > volcanic glass ⩾ hornblende ⩾ pyroxenes. Amorphous material is dominant in the clay fractions of all soils. Crystalline clay minerals such as montmorillonite, vermiculite, Al-chlorite, halloysite, kaolinite, and imogolite are also present.