Epitaxy in the Crystallization of Feldspar Gels and Glasses
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other feldspars do not exceed 5.5% along 'a' axis or 1.6% along Vband Vcaxes [2]. Thus there is a strong likelihood that one feldspar can grow epitaxially on the surface of another feldspar crystal. Using this idea, we succeeded in crystallizing Na-, K- and Ba-feldspar glasses or gels into corresponding feldspar crystals by isostructural seeding [14-17]. Continuing these studies, the present work further discusses and summarizes the details of crystallization mechanisms involved in the crystallization of feldspar gels or glasses upon seeding. EXPERIMENTAL PROCEDURE Stoichiometric feldspar glasses, monophasic and multiphasic gels of composition corresponding to alkali feldspars (NaAlSi 3 O8 and KA]Si3 O8 ) and alkaline earth feldspars (CaAl 2 Si 2 Og, SrAl2 Si2 0 8 and CaAl 2 Si 2 O8 ) were synthesized. These gels and glasses were then seeded with 5 wt% of different crystalline sols, including isostructural seeding with crystalline feldspars of NaAlSi 3 O 8 , KAMP&O 8 CaAl 2 Si 2 O8 , SrAl 2 Si2 0 8 and BaAl 2 Si 2 Og, and nonisostructural seeding with LiAlSi3 O8 (r3-spodumene solid solution), TiO 2 (rutile) and ZrO 2 (tetragonal phase). Details of preparation of glasses, gels and seeds have been given elsewhere [14-I17]. The studies of epitaxial crystallization mechanism were conducted by comparing the crystallization behaviors and microstructures of the feldspar gels and glasses before and after seeding under the same heat treatment conditions. The crystallization products were mainly determined by X-ray diffraction (XRD) analysis. Microstructures of epitaxy upon seeding were examined with Scanning electron microscope (SEM), and energy dispersive spectrometry (EDS). RESULTS AND DISCUSSION Epitaxy in Alkali Feldspar Glasses and Gels 5 wt% of crystalline NaAISi 3 Og. CaAI2 Si 2 O8, SrAI 2 Si2 O8 and ZrO 2 were mixed with
albite (NaA]Si 3 O8) glass and heat treated at temperatures of 8000 to 1200'C. No crystalline albite was obtained in unseeded or nonisostructural ZrO 2 seeded albite glass, whereas isostructural NaAISi 3 O8 , CaAJ2 Si 2 O8 and SrAJ2 Si 2 Og seeded albite glasses crystallized to high albite at temperatures of 850'C up to the melting point [14,15]. SrAl2Si 2 0 8 crystal is an isostructural but nonisocompositional seed for albite glass. Figure 1 shows XRD patterns of the SrAl2Si 2 O8 seeded albite glass samples heated at 950'C for various times. The small peaks in the unheated samples are due to the prior addition of 5 wt% SrAl2Si 2Og seed. These peaks disappeared after reacting with the glassy matrix and maximum crystallization of albite occurred after heating for 8 hours at 950'C. Upon further heating, the peak intensity increased slightly probably due to grain growth of the crystals. This crystallization behavior is similar to that of CaAl2Si 2 Og seeded albite glass [15]. Similar phenomena were also found in isostructurally seeded KA]Si 3 O 8 multiphasic gels [16]. The pure glass, mono- and multiphasic gels of KAJSi 3 O8 crystallized mainly to leucite (KAIM2 O6 ) and no feldspar phases crystalliz
