NH sub(4) for K substitution in dioctahedral mica synthesized at 200 degree C

Synthesis of intermediate K-NH sub(4) dioctahedral micas from kaolinite had been carried out using two chemical systems K sub(2)O-(NH sub(4)) sub(2)O-Al sub(2)O sub(3)-SiO sub(2)-H sub(2)O (1) and K sub(2)O-(NH sub(4)) sub(2)O-Al sub(2)O sub(3)-SiO sub(2)-HCl-H sub(2)O (2). We have used variable NH sub(4):K ratios, with the aim of testing the existence of a possible miscibility gap between the NH sub(4)- and the K-end members at 200 degree C. Reactions in system (1) mainly produced zeolites. In system (2), a complete series of mica (except for the molar NH sub(4):K ratio = 0.0:1.0), were obtained. The results based on the X-ray diffraction patterns and Fourier Transform Infrared spectra indicated, in contrast with previous works, that a linear correlation between the average NH sub(4) content in mica and either the basal spacing or the area of the main NH sub(4) infrared band does not exist in the series studied. Thus, the direct estimation of the NH sub(4) in mica from these methods seems questionable. Indeed, the current NH sub(4) content in mica appears to be controlled by several factors including, in addition to the initial NH sub(4) content in the system, the presence of poorly crystalline phases, which is favoured in NH sub(4)-poor systems, and the equilibrium between the solid products and the solutions. Data for discrete mica particles obtained from the transmission-analytical microscopic study suggested that a miscibility gap exists in the range ~((NH sub(4)) sub(20) K sub(80)-((NH) sub(4)) sub(60)K sub(40) at 200 degree C. This is analog to the gap observed in solid solutions of paragonite and muscovite.