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This article in SSSAJ

  1. Vol. 23 No. 2, p. 101-105
    Received: July 30, 1958
    Accepted: Sept 19, 1958

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Constancy of the Sum of Mica Unit Cell Potassium Surface and Interlayer Sorption Surface in Vermiculite-Illite Clays1

  1. O. P. Mehra and
  2. M. L. Jackson2



The total unit cell planar specific surface was computed as the sum of planar sorption surface (by a glycerol gravimetric method) and the mica unit cell interplanar surface (corresponding to the K). This sum was found to be constant for a given unit cell formula weight, averaging 773 m.2/g. with a standard deviation of ±12.7 or about ±2%. For example, a Colorado vermiculite had 1.63% K2O equivalent to 16.3% mica residue with a unit cell interplanar surface of 124 m.2/g.; this added to the measured 631 m.2/g. of planar sorption surface (glycerol sorbed on expanded or cleavage planes) gives a total planar surface of 755 m.2/g. Similarly, for coarse clay from Fithian, Illinois, the 5.61% K2O is equivalent to 56.1% of illite, with unit cell interplanar surface of 426 m.2/g. This, added to 235 m.2/g. of measured planar sorption surface, gives a total of 661 m.2/g. which when corrected to exclude 15% unexpanded minerals (kaolinite and chlorite) gives a total planar specific surface of 775 m.2/g. Wyoming montmorillonite had 803 m.2/g. of planar specific surface, comparing well with the theoretical, 808 m.2/g. This principle of unit cell planar specific surface constancy of 2:1 layer silicates shows that the mechanism of K release from mica is by cleavage, and gives an accurate tool for analysis of the rather generally occurring interstratified mixtures of expanding 2:1 layer silicate minerals with micas in soils and sediments.

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