Constancy of the Sum of Mica Unit Cell Potassium Surface and Interlayer Sorption Surface in Vermiculite‐Illite Clays

Abstract

Abstract 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% K 2 O 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% K 2 O 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.