Ordered - Disordered Stacking Structure Along the c-Axis in Calcium Rare -Earth Fluorocarbonate Minerals

摘要

The ultrastruetural characteristics of members of the calcium rare-earth fluorocarbonate mineral series from Sichuan Province, Southwest China, have been studied using selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). Microstructural and ultrastructural information of this mineral series has been obtained from derived polycrystal of parisite, such as crystal structural defects in parisite and regular mixed-layer structures BniS|,(m>n), antiphase domains and disordered domains in the two new polytypes B2S-2H and 12R, syntactic inlorgrowlhs and disordered stacking among the structural unit layers of BroSn (m>n) type along the c- axis direction . Five new, regular, mixed-layer structures of the BmSn(m>n) type with long periods and difftrent stacking sequences, D(,S2, D19S10, BuSs, B2S and B2S2 were found, their structural symmetry, cell parameters, chemical formulas of crystal and stacking models of structural unit layer were determined . The results of this investigation reveal the- complexity of the structure in syntactic polycrystals of such a mineral series, The new, regular, mixed-layer structures and the new polytypes are composed of structural unit layers of the two end elements of this mineral scries - bastnacsite(B) and synchisite(S) with ordered stacking in varying proportions along c axis. The different, regular, mixed-layer structures and polytypes have various compositions and structural stacking models.