| Abstrakt: |
A fragipan developed in a late‐Wisconsinan, loess‐derived soil was studied in detail to determine if its brittleness could be related to a specific chemical bonding agent. Three approaches were used in the study. The first involved an attempt to create fragipan brittleness and consistence by adding solutions containing Fe, Al, or Si to non‐fragipan, loess material. Secondly, a destructive approach utilizing various dissolution techniques was used to extract fragipan material under vacuum in an attempt to selectively remove the cementing agent. Lastly, the fragipan fabric was observed and analyzed in situ by scanning electron microscopy (SEM) and energy dispersive X‐ray analysis (EDXRA). Solutions containing Si or Al added to acid, nonfragipan loess material increased the rupture strength. Na‐citrate‐bicarbonate‐dithionite (CBD) and Na‐citrate‐dithionite solutions destroyed fragipan peds under vacuum and eliminated the slaking properties of the residue. Pyrophosphate also destroyed the ped fabric and dispersed clay, but the residue retained its ability to slake in water. Differential infrared spectroscopy and differential thermal calorimetry revealed that CBD selectively removed an inorganic fraction unique to the fragipan. SEM study demonstrated that bridges occurred between silt grains in the fragipan and EDXRA revealed that these bridges contained considerably more Si than identifiable argillaceous plasma in either the fragipan or nonfragipan horizons. In conclusion, the key bonding agent in the fragipan studied appears to be a hydrous aluminosilicate, possibly associated with clay, which formed bridges between skeletal grains. |
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