In calculating the formulas of members of the montmorillonite group all the magnesium is generally assigned to the Al layer. However, determination of exchangeable cations in 14 montmorillonitic clays, 10 of them the identical specimens for which analyses were quoted by Ross and Hendricks (U. S. Geological Survey, Prof. Paper205-B), showed that in all of them some of the magnesium was exchangeable, the amount ranging from 0.11 to 1.62 per cent. In two samples exchangeable magnesium amounted to about J of the total magnesium. These results indicate that the presence of exchangeable magnesium in the montmorillonitic clays is more common than is generally recognized.

Ross and Hendricks, assigning all magnesium to the Al layer in calculating the formulas of montmorillonites, found that the number of ions in octahedral coordination ranged from 2.02 to 2.24. However, a recalculation of the formulas of some of these samples on which exchangeable cations were determined showed that, when total magnesium is corrected for exchangeable magnesium and only non-exchangeable magnesium is assigned to the Al layer, and when the determined rather than an average cation-exchange capacity is used, the number of cations in octahedral coordination does not exceed 2.02, even in those samples in which the number of cations is as much as 2.13 when all the magnesium is assigned to the Al layer. Apparently, in montmorillonite then, substitution of Mg for Al in the Al layer is ion for ion, with no extra ions added in an effort toward internal compensation of charge.

By the Marshall method of calculation, the number of atoms in the octahedral layer in excess of 4.00, when all the magnesium is assigned to the octahedral layer, permits an estimation of the amount of exchangeable magnesium present and, from this, an estimation of the cation-exchange capacity of the sample. On the other hand, when only non-exchangeable magnesium is assigned to the octahedral layer, the number of atoms in this layer is very close to the theoretical 4.00, and the charge on the cell checks the charge of the external cations and the determined cation-exchange capacity, which it does not do when all the magnesium is assigned to the octahedral layer.

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