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Differential Dissolution Analysis of Clays and its Application to Hawaiian Soils
|Title:||Differential Dissolution Analysis of Clays and its Application to Hawaiian Soils|
|Abstract:||Analyses of soil clays are carried out by several independent physical and chemical methods because each method has limitations. Differential dissolution methods are being developed based on crystal structure characteristics of minerals, the basis for mineral classification. Dissolution analysis provides not only a means of obtaining the chemical composition of labile minerals but also provides samples free of these materials for analysis of more resistant minerals. Development of dissolution techniques involves selection of specific reagents, establishment of optimal conditions for reaction, and recognition of their limitations.|
The objectives of this investigation were to develop a method for differential dissolution of amorphous constituents from soil clays and of the kaolinite plus halloysite components of crystalline soil clays.
The results may be summarized as follows:
Half normal HaOH solution was chosen because of its rapid and extensive dissolution of both silica and alumina, the two major constituents of clay minerals. Use of 2% Na2CO3 solution gave less complete dissolution.
Boiling for as little as 2.5 minutes in the NaOH solution dissolved a large quantity of allophane-like materials and free silica and alumina from clays of some montmorillonite-rich soils of Hawaii, provided the ratio of clay to solution volume was kept less than 100 mg to 100 ml. Subsequent dithionite-citrate-bicarbonate treatment removed the released iron. Various typical allophane specimens were completely dissolved by this procedure.
Reprecipitation of dissolved silica occurred with prolonged digestion or higher sample to solution ratio. Digestion for 80 minutes dissolved 50% of Georgia kaolinite and 25% of Wyoming montmorillonite, but only a small quantity of crystalline minerals dissolved during the short digestion period adopted. Halloysite specimens appeared somewhat unstable, however. Gibbsite dissolved very readily together with the amorphous constituents.
Marked improvement of x-ray diffraction patterns of clays resulted after the removal of amorphous materials. A few percent of crystalline components were identified in the clay fraction of allophane from soils and deposits.
After dihydroxylation treatment, halloysite and kaolinite became amorphous and were dissolved by the same differential procedure. Dissolution of kaolinite against preheating temperature was essentially identical to the dehydration curve. A sharp break occurred at 500C. Some temperature variation in a furnace space is common and thus 4 hours of heating at 525C should be employed for dehydroxylating kaolinite. Heating weight loss data also correlated closely with dissolution. Heat-stable (aluminous) montmorillonite and chlorite were only slightly dissolved. Iron-rich vermiculite and nontronitic montmorillonite were partially dehydroxylated by the heating process and dissolved accordingly. The process of heating lowered the solubility of amorphous materials and gibbsite.
After dihydroxylation, the ferruginous clays of Hawaiian montmorillonitic soils released an additional amount arising from kaolinite and halloysite, helpful in determining these minerals. A considerable amount of iron from nontronitic montmorillonite was extracted also which necessitated a refinement in the allocation.
Digestion in 2 normal HCl had very little effect on kaolinite samples while the same treatment dissolved Colorado vermiculite and a nontronitic soil montmorillonite completely. Relatively labile halloysite from Hawaii was highly susceptible to dissolution by such a treatment. The removal of heat-unstable 2:1 layer silicates was desirable to concentrate the kaolinite and halloysite. Photo-chemical reduction of iron in 0.125 molar oxalate buffer was less severe than the HCl treatment but showed no specific effect on iron-rich minerals.
The proposed differential dissolution procedure using a NaOH solution is rapid, simple and fairly quantitative, through simple analysis of dissolved elements. The removal of amorphous constituents and kaolinite plus halloysite should greatly improve the accuracy of analysis of the remaining components by the conventional methods, and is particularly helpful in evaluation of the 7 A diffraction spacing of clays.
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Ph.D. - Agronomy and Soil Science|
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