THE INFLUENCE OF DIFFERENT PRETREATMENT METHODS ON GRAIN SIZE DRISTRIBUTION OF RED WEATHERING CRUSTS

As to the pretreatment methods of grain size analysis for red weathering crusts, at present there is a great discrepancy,and it influences the reliability of analytic results to some extent.This paper adopted different pretreatment methods to measure the grain size of red weathering crust samples in karst areas,southwestern China,and the results showed that they have apparent effects on the grain size data.As to samples rich in gibbsite,boiling with dilute HCl solution may disaggregate cementation of disassociative Al-oxide and enable grain size to be dispersed well,i.e.,method B is good.However,as for samples rich in smectite,owing to destroying crystal structure of smectite and silicon-colloid unmixing through acting with HCl solution,it resulted in cementation of mineral particles,so method A is good.Other kinds of clay minerals have no obvious sensitivity for different pretreatment methods.When clay content is used as the index for weathering intensity of red weathering crusts and type devision of red soils,different pretreatment methods will influence the accurate explanation of the results.Therefore,to select an appropriate pretreatment method is very necessary.With regard to genesis research on red weathering crusts underlying carbonatite,when grain size frequency distribution curves are used as indices,different pretreatment methods have no influence on source indication;but when clay content and ratio of silt to clay are used,the difference will likely effect research results.