A comparative study was conducted to evaluate chemical dissolution techniques for separating non-crystalline components from minerals. The techniques examined included six separate treatments with NaOH, Na
2CO
3, alkaline Tiron, dithionite-citrate-bicarbonate (DCB), acid ammonium oxalate and sodium pyrophosphate. These dissolution techniques were applied to compositionally different non-crystalline silicates, fractionated samples of goethite, hematite and gibbsite, and synthetic crystalline and non-crystalline iron (or aluminum) oxyhydroxide minerals. Their efficiency and specificity were discussed on the basis of the results obtained.
The extraction capacity changed with not only the type of dissolution techniques but also the composition of non-crystalline components to be dissolved. For a composition of Si-Al system, the NaOH, Na
2CO
3 and Tiron methods indicated little selectivity with regard to the two elements. For a Si-Al-Fe-Mg system composition, however, the NaOH and Na
2CO
3 methods extracted preferentially Si and Al. As anticipated, a strong selectivity for Fe was always observed with the DCB method. The Tiron method showed the least selective extractions of these elements among the methods examined. For Fe-oxides, Fe-oxyhydroxides, Al-oxyhydroxides and Al hydroxides, the Tiron method extracted as much Fe and Al from respective minerals as the oxalate method did except for a few abnormal cases. As particlesizes of hematite, goethite and gibbsite decreased, amounts of extractable Fe and Al increased. This should be borne in mind when extraction techniques are applied to samples containing fine crystalline particles. Although the chemical dissolution techniques are conventional and operational, they can be very useful. If techniques are appropriately chosen depending on the case and the resulting extraction data are then carefully interpreted, unique information concerning non-crystalline components should be available.
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