THE NAALSIO4 NEPHELINE-CARNEGIEITE SOLID-STATE TRANSFORMATION

The reconstructive high-temperature transformation of NaAlSiO4 nepheli ne to carnegieite, stuffed derivatives of the SiO2 phases tridymite an d cristobalite, has been studied by light microscopy and X-ray precess ion photographs. X-ray precession patterns of single crystals heat-tre ated between 1543 K and 1593 K yielded an oriented transformation of N aAlSiO4 nepheline to carnegieite with [111]CA\\[001]NE and [hh2h]CA\\[ 100]NE (cubic setting of carnegieite (CA), and orthohexagonal setting of nepheline (NE)). Two equivalent carnegieite orientations occur, bot h having [111]CA\\[001]NE in common, corresponding to transformation t wins with {111}CA as twin plane. The oriental relationships between ne pheline and carnegieite are similar to those observed in the high-temp erature transformation of tridymite to cristobalite. However, the tran sformation mechanisms are believed to be different. Thermally activate d Na and weakening of the tetrahedral Al-O-bonds produce quickly migra ting non-bridging O-atoms which act as conversion nuclei, accelerating strongly the transformation process in comparison to SiO2.