DISSOLUTION OF TRIOCTAHEDRAL LAYER SILICATES BY AMMONIUM OXALATE, SODIUM DITHIONITE–CITRATE–BICARBONATE, AND POTASSIUM PYROPHOSPHATE

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Abstract
The effects of NH4-oxalate (pH 3), Na–dithionite–citrate–bicarbonate (pH 7.3), and K-pyrophosphate (pH 10) on various size fractions of biotite, chlorite, muscovite, and illite minerals were investigated. The data showed that the release of mineral cations by the three chemical extractants increased with decreasing particle sizes. The dissolution of layer silicates was slight and gradual in the particle size range of 100–5 μ, increased considerably for 5–2 μ and markedly below 2 μ. In general, NH4-oxalate dissolved the maximum percentage of Fe, Mg, and Al, and K-pyrophosphate extracted the least amount. Approximately 18.8% of original Fe, 15.5% of Mg, and 11.3% of Al were extracted from 0.2–0.08 μ-size biotite by the oxalate procedure. The corresponding values for the same size fraction of chlorite were: 23.2% Fe, 18.5% Mg, and 13.5% Al. In view of the data obtained it is suggested that the use of NH4-oxalate as an extractant for amorphous Fe and Al may have limitations for soils containing trioctahedral minerals. Instead, K-pyrophosphate, which has little effect on crystalline minerals and also has the advantage of extracting organic matter-complexed Fe and Al, may be more suitable.
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