Physical,chemical and mineralogical properties of weathering crust of granitic rocks distributed in Antioquia,Colombia.

Abstract

The weathering crust of granitic rocks in Antioquia, Colombia, can be divided into eleven zones. Since zone IX that has been formed by wind-bourne loess is not a weathering crust in situ, it was excluded from the study. Zones X and XI, being soil, were also excluded. The grain distribution was mechanically analyzed for the samples from zones V-VIII. The change from “masa” A (zone V) to “masa” B (zone VI) is characterized by transformation of pebble to sand, the change from “masa” B to red “masa”(zone VII) is characterized by transformation of silt to clay and the change from red “masa” to “shimofuri” clay (zone VIII) which does not show noticeable alteration of the silt-size grains to clay-size grains is characterized by transformation only of sand to silt.Mineralogical composition of weathering zones are identified by the X-ray diffraction method. The presence of quartz was confirmed in all the weathering zones. K-feldspar begins to alter from weathered granite A (zone III) and completely disappears in “masa” A. Plagioclase and biotite disappearsalmost completely in the initial stage of weathering. Of the secondary minerals by the weathering action, chlorite, illite, halloysite and gibbsite are seen. The existence of kaolinite is observed very conspicuously in red “masa” and “shimofuri” clay.Chemical analysis was carried out by X-ray fluorescence spectrometer. The transfer of CaO and Na2O shows the tendency of decrease in a convex-shaped curve in the weathering crust in Japan but in a concave-shaped curve in Colombia. The transfer of K2O and MgO shows almost no change in the weathering crust in Japan but a tendency of rapid decrease in Colombia in “masa” B. SiO2 decreases with the progression of weathering but resumes an increase in red “masa”. This also well corresponds with the presence of gibbsite until “masa” B which transforms to kaolinite in red “masa”. Although literatures have shown that the progression of weathering induces reduction of SiO2 with the result of final formation of gibbsite, the new finding obtained in the present study shows that further progression of weathering to red “masa” or “shimofuri” clay induces, on the contrary, infusion of SiO2 with the consequence of kaolinite formation from gibbsite.