THE ORIGIN OF INCOMMENSURATE STRUCTURES: PAMC AND THE VERY STABLE MINERAL MULLITE

摘要

The atomistic origin of incommensurate modulated structures are reviewed briefly. PAMC and mullite both show how incommensurate structures are good structures, optimising different contributions to stability. The simplest to understand is probably propyl ammonium manganese chloride (PAMC). The mineral mullite Al2(Al2+2xSi2-2x)(>10-x is also considered. It is very stable in two senses: firstly it melts on heating before it disorders and secondly its existence at all as a stable phase in the SiOj/AtjC^ phase diagram is remarkable given the extreme stability of the end members. Other stable alumino-silicates will be mentioned including the plagioclase feldspars.