CHANGES IN SWELLING CHARACTERISTICS OF FLUORINE MICAS BY HEAT-TREATMENT

Sodium fluorine micas having variable layer charges [Na(x)Mg(3-x)Li(x)(Si4O10)F2; x = 1.0, 0.8, 0.6] and their cation-exchanged forms (Ba-, Ni-type) were heated at high temperatures (approximately 1000-degrees-C) under various conditions such as i) in air, ii) in fluoride-containing mixture, and iii) in a sealed platinum vessel, and swelling characteristics and cation exchangeability of fluorine micas were investigated by XRD, IR and AAA methods to clarify the effects of heat treatment on swelling tendencies. The content of exchangeable Na+, the ratio of nonswelling to swelling phase and/or low-hydration phase to high-hydration phase increased with increasing temperature and heat-treating time for Na-micas having high charges (x = 1.0, 0.8), showing reduction in swelling tendencies by heating. However, thermal decomposition of micas occurred above 700-degrees-C in air and above 800-degrees-C in fluoride-containing mixture, respectively, while it was prevented in the sealed vessel at 700 approximately 1000-degrees-C. When Na-micas (x = 1.0, 0.8) and their Ni-exchanged forms were heated in the sealed vessel at 1000-degrees-C for prolonged time and above 800-degrees-C, respectively, the infrared absorption band (ca. 980 cm-1) ascribable to Si-O(b) (basal oxygen) vibration shifted to lower frequencies for the former and to higher frequencies for the latter, respectively. This shows that the interlayer structure of mica was rearranged through the migration of interlayer cations into hexagonal holes of tetrahedral sheets; the migration is easier for smaller cation than for larger cation and the buried cations in hexagonal cavities lose their hydration properties. The changes in b-dimension of the mica lattice were also observed with increasing temperature, supporting above-mentioned concepts in the mechanism of loosing swelling properties for heated micas. Heat treatment was also effective for making Ba-exchanged mica nonswelling, however, the mechanism is different from the one described above because the largest Ba2+ cation can establish a similar stable coordination interlayer polyhedron to that of the typical nonswelling K-mica. On the contrary, heat treatment for Na-micas having low charge (x = 0.6) was less effective because substantial unstableness results from the weaker interlayer bonding.