Mica Surface Morphology Changes During Weathering

Electron microscopic observations of mica surfaces by the carbon replica technique indicate that weathering, a major process of K release in soils, proceeds at the (hkO) fracture edges and at the crystallographic dislocations distributed on the (001) cleavage faces of mica crystals. Fresh cleavage surfaces of a paragonite with muscovite inclusions show discontinuity of the layers and marginal rolling at edges of K mica cores 3 to 5µ in diameter. A closely intergrown mosaic of the two micas is indicated rather than a true solid solution of the K,Na. Much of the interlayer Na in paragonite readily exchanges for K on treatment with 1 N KCl solution and the surface assumes the smoother morphological features of K mica. Fresh cleavages of muscovite and margarite show some folding and scrolling of layers at dislocation edges. Artificial laboratory weathering of biotite to vermiculite by treatment with MgCl 2 produces surface morphology similar to that of natural vermiculites. Removal of K from biotite with tetraphenylboron causes the surfaces to crack open, yielding fresh (hkO) fracture edges. Weathering with NaOH or HCl produces characteristic surface changes. Treatment of biotite with HF for one minute produces humps scattered at 0.1 to 1µ intervals over the (001) cleavage surfaces. Desilication for a longer period produces characteristic etch pits and corroded surfaces with serrated edges. These surface studies provide a new model of mica weathering at (hkO) edges formed on the (001) cleavages, and shows layer flexibility on cation exchange which greatly aids in explanation of K release and fixation in soils.