Identification of Clay Minerals in Soil: The Effect of Sodium-Pyrophosphate
- J. A. I. Omueti and
- L. M. Lavkulich
The effect of Na4P2O7 on soil clay minerals, layer silicate clays, and chloritized montmorillonite (synthetic) was investigated by x-ray diffraction. The soil samples were treated with 30% H2O2, boiled, and water-washed by centrifugation. The resulting organic-free solids were dispersed as follows: shaken with 0.1 M Na4P2O7 · 10H2O for 16 h only; treated with dithionite-citrate-bicarbonate (DCB); and treated with DCB followed by 0.1 M Na4P2O7 · 10H2O. The 0.1 M Na4P2O7 · 10H2O treatment caused the peak at 1.403 nm in the soil samples to become enlarged, sharp, and move to 1.339 nm. Other sharp peaks were also produced at 0.666, 0.637, 0.590, 0.561, 0.444, and 0.334 nm. These peaks were not observed in the x-ray diffractograms of kaolinite and halloysite, but those of montmorillonite, chloritized montmorillonite, and chlorite were very similar to those of the soils. Heat treatment at 100°C destroyed the 0.637-, 0.590-, and 0.561-nm peaks in some of the soil clay samples. At 300° the remaining peaks disappeared suggesting that sharp peaks resulting from the 0.1 M Na4P2O7 treatment may be the result of mineral hydration. Some of these new peaks from 0.1 M Na4P2O7 treatment of soil clays, the 0.334-nm peak for example, can be mistaken as indicative of quartz in samples that do not contain this mineral. Thus the continued use of this reagent for mineralogical studies is not recommended.
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