STRUCTURAL CHANGE IN CELADONITE AND CIS-VACANT ILLITE BY ELECTRON RADIATION IN TEM

摘要

High-resolution transmission electron microscopy (HRTEM) images of two dioctahedral micas, celadonite and cis-vacant (cv) illite, were examined in detail to understand the effects of electron radiation on their structures during image acquisition. Celadonite, a dioctahedral mica with Fe and Mg as major octahedral cations, originally has a trans-vacant (tv) octahedral sheet but the contrast in the high-resolution transmission electron microscopy (HRTEM) images indicates a different cation distribution in the sheet. Furthermore, the β angle for the 1 Mpolytype derived from the HRTEM images is ～98.5°, which is considerably smaller than that (～ 100.5°) reported for celadonite. In previous works, cation migration from the tv to cv-like configurations and a decrease in the p angle after dehydroxylation of celadonite/ glauconite by heating were reported. The same phenomenon, dehydroxylation and subsequent cation migration, probably occurs by electron radiation in TEM. However, the new cation-distribution models derived from HRTEM images along the [100] and [110] directions are not in agreement. On the other hand, the contrast in a number of HRTEM images from an illite specimen in which cv-illite is dominant is the same as that for the tv-dioctahedral layer. This result is also interpreted as cation migration accompanied by dehydroxylation in TEM, as reported in heated cv-illite. The increased β angle (～ 102.5°) from that in the natural state (101.5°) estimated from the HRTEM images also supports this interpretation. This phenomenon is a large obstacle to the investigation of phyllosilicates containing Al-rich cv and Mg,Fe-rich tv 2:1 layers, using HRTEM.