Cation Exchange Capacity and Weathering of Muscovite Macroflakes

Abstract

AbstractThe cation exchange capacity (CEC) of large (3‐ to 7‐cm2) muscovite macroflakes, determined with 45Ca, was 8.6 × 10‐7 meq/cm2 of geometrical surface, a value greater than the theoretical one of 3.5 × 10‐7 meq/cm2 calculated from the structural formula. The greater than theoretical CEC was attributed to exchange in areas of growth planes made accessible to the solution by (hkO) cleavage and by charged particle tracks. Similar values were obtained for BaEC (9.7 × 10‐7 meq/cm2) and CsEC (8.7 × 10‐7 meq/cm2). The excess of BaCl2, CsCl, or NaCl was not completely removed, whereas excess CaCl2 was removed, by five 100% methanol washings, as indicated by radiochemical measurements. The presence of excess BaCl2 and absence of CaCl2 were confirmed by electron micrographs of platinum‐carbon replicas of the muscovite cleavage surfaces, the former being removed by subsequent water washings. Hydrolysis of exchangeable Na was appreciable during water washings in the NaEC determinations, resulting in a considerably lower measured NaEC (1.1 × 10‐7 meq/cm2), but little hydrolysis of exchangeable Ba or Cs occurred in water washings. Electron micrographs showed that the muscovite (001) cleavage surfaces had been etched during extensive salt treatments and that a weathering product had formed on the surfaces. The latter material was insoluble in water, but was removed by treatment with Na citrate‐NaHCO3, indicating a sesquioxidic composition. The CEC of muscovite flakes was unaffected by the etching and the sesquioxidic weathering product.