A B S T R A C T INTERLAYER K in muscovite, biotite, phlogopite, illite and vermiculite-hydrobiotite samples was replaced by cation exchange with Na. The rate and amount of exchange varied with the mineral and the level of K in solution.Essentially, all the K in muscovite, biotite, phlogopite and vermiculite was exchangeable when the mass-action effect of the replaced K was reduced by maintaining a very low level of K in solution. The time required for this exchange varied from < 10 hr with vermiculite to > 45 weeks with muscovite. Only 66% of the K in the illite was exchangeable under these conditions. When the replaced K was allowed to accumulate in the solution, the amount of exchange was determined by the level of K in solution required for equilibrium. These levels decreased with the degree of K-depletion and with the selectivity of the mica for K. The order of selectivity was muscovite > illite > biotite > phlogopite > vermiculite. Decreasing the K in solution from 10 to 7 ppm increased the exchangeable K in biotite from 30 to 100%. A K level of only 0.1 ppm restricted the exchange of K in muscovite to 17%.A decrease in layer charge was not required for K exchange, but a decrease did occur in K-depleted biotite and vermiculite. Muscovite with the highest layer charge (247 meq/100 g), least expansion with Na (12.3 Ä), and least sensitivity to solution p H had the highest selectivity for K and the slowest rate of exchange. The K in vermiculite was the most readily exchangeable.

1. Spikes in molluscan bursting cells are followed by depolarizing afterpotentials (DAPs) which are not seen in nonbursting cells in the same ganglia. DAPs from successive spikes sum to provide a depolarizing drive capable of sustaining multiple discharge. 2. Subthreshold depolarization activates a DAP-like process in bursters. 3. DAP amplitude increases as the cell is hyperpolarized beyond the potassium equilibrium potential. The amplitude is not changed by intracellular iontophoresis of TEA-Cl. DAP amplitude is reduced by 39% after a 10-min exposure to CA++-free saline, and by 66% after 10 min in 4% Na+ (Tris substituted) saline. The amplitude is unchanged by exposure to K+-free saline. 4. During repetitive stimulation the amplitude of the summed DAP declines. 5. It is concluded that DAP results from a slowly decaying component of Ca++ and Na+ permeability.

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When large flakes of edge‐weathered biotite were air‐dried, some of the silicate layers were forced apart while others were brought closer together. Since the space between the separated layers developed into visible cracks that penetrated the inner mica core, the dry flakes provide visual evidence for a mechanism by which interlayer K can be released when soils are dried. The collapse of part of the lattice can give rise to a simultaneous occurrence of K fixation.