CRYSTALLIZATION MECHANISM OF MAS-OSUMILITE WITH COMPOSITION MG2AL4SI11O30 FROM GLASS

MAS-osumilite is a metastable double-ring silicate which is attractive for use in glass-ceramics because of its low thermal expansion coeffi cient. The phase can be crystallized from glass powders of MAS-osumili te composition containing small amounts of BaO. The nucleation and cry stallization process has been investigated by means of TEM and SEM. Cr ystallization starts with the precipitation of beta-quartz(ss) of comp osition MgAl2O4.nSiO2. BaO concentrates in a residual glass phase betw een the beta-quartz(ss) crystals. In a second stage small amounts of B a-osumilite (BaMg2Al6Si9O30) nucleate and crystallize from this glass phase. In a third stage BaO-free MAS-osumilite crystals are formed by epitactic growth on the Ba-osumilite surfaces and grow into adjacent b eta-quartz(ss) grains. Two factors are essential for the suppression o f undesired further phases to develop a monophase (besides some Ba-osu milite) MAS-osumilite glass-ceramic: a residual glass phase with a com position close to Ba-osumilite, and beta-quartz(ss) crystals having a composition close to that of MAS-osumilite.