September 2000 Thermochemical Reactions and equilibria between Fluc The interfacial compositions of the stabilized celsian and fluorokinoshitalite establish the thermochemical conditions for one of perhaps several two-phase equilibria. Affected by the use of pecific starting compositions and the diffusion dynamics, many fluorokinoshitalite equilibrium tie lines could exist between a Ba-K solid-solution feldspar and a Ba-K solid-solution mica. Tables I and ll display the experimentally measured interfacial compositions of all atomic species in the system at 1300C (all compositional analysis was conducted at interfacial regions that showed minimal effects of mica anisotropy). Experimentally measured bulk compositions interface separation also are presented for comparison. Although the response observe at 1200 C was much less dramatic(interdiffusion of K+and Si+ for Ba- and Al extended away from the original interface only 40 um into the mica and 25 um into the feldspar), the nterfacial compositions, shown in Tables Ill and Iv, are quite similar to those measured at 1300C. The apparent insensitivity of the interfacial compositions to the processing temperature suggests stabilized celsian that thermochemical equilibrium can be established over a wide temperature range with a narrow range of compositions, despite the non-unique compositional conditions that generally describe a two-phase solid-solution equilibrium. 24,25 Knowledge of these thermochemical conditions introduces the opportunity for the design of globally stable fluoromica interphase/silicate matrix interfaces Fig. 2. BEI micrograph of a stabilized celsian/fluorokinoshitalite reaction K(+Sr)-stabilized celsian and fluorokinoshitalite are not stable interface processed at 1300.C. The fluorokinoshitalite substrate separated against each other at 1400C. Fine particles of stoichiometric from the stabilized celsian substrate on final polishing. Gradual contrast celsian(BaAl, Si,O,) and spinel(MgAl, 04) were the only addi- hanges in both phases( the brighter phase is celsian, and the less-bright tional phases observed along the interface. In addition, large(2 placing Ba+. Fluorokinoshitalite grains oriented with basal planes um)bubbles were produced along the exterior edges of the rallel to the interface show exceptionally bright contrast, in comparison fluoromica substrate, indicating the volatilization of gaseous flu- to grains oriented perpendicular to the interface, indicating that these orin(F2). The formation of celsian and spinel phases, as well as former grains do not participate easily in the ion exchange gas bubbles, strongly suggests that interfacial reaction occurred via the combination of MgO from the fluoromica with slight alumina excesses from the feldspar-based matrix, as articulated by eqs. 3) Ba2+ concentration correspond to a fluoromica grain that was and(4). These results again suggest that Mgo buffering may be aligned parallel to the interface), the coupled nature of this process highly effective in achieving thermodynamic equilibrium between C g represented clearly by the simultaneous diffusion of K and alumina-rich stabilized celsian and fluorokinoshitalite. even at cations into the fluorol ations into the feldspar. Charge neutrality of the interdiffusing species is The thermochemical response between stabilized celsian and maintained, des p a concentration gradientof the Al+ species. Interdiffusion Figure 4 shows micrographs of such an interface that has been extended out as far as 100 um away from the interface in the processed at 1300%C. The gradual change in contrast within the mica and -50 um in the feldspar at this temperature, which is fluorophlogopite, visible in the bEl image in Fig. 4(a), again consistent with the information available from BEl (Fig. 2) indicates compositional gradients across the interface, however, in Furthermore, the coupled cationic exchange between these two this case, high-Z elements increase in concentration toward the materials is isomorphous; i.e., both phases are stable against each interface. EDS linescans(Fig. 5)confirm the interdiffusion of the other, but their compositions change. As illustrated in Fig 3(b), the K+ species from the mica and the Ba2+ species from the feldspar mpositional profiles of both F and Mg- cations remain (the coupled counterparts, Si" exchange for Al, although not constant throughout the fluoromica at levels consistent with bulk thown in this figure also were observed). Similar to the reaction fluorokinoshitalite compositions, but then decrease abruptly to between stabilized celsian and fluorokinoshitalite, uphill diffusio zero at the original stabilized -celsian/fluorokinoshitalite interface observed again, except this time for the Si species demonstrating the phase stability of the fluoromica On closer examination of the 1300%C reaction, fine particles A sharp change in Sr+ concentration at the interface also is (<5 um) of stoichiometric spinel (MgAl,O4)and leucite evident in Fig. 3(b), which suggests that the mobility of St (K[AISizJO,)were observed along the interface via WDS analysis cations is much slower than that of Ba- cations in the mica(i.e, Figure 4(b) shows a SEl image of the reaction, where the dark D5r+< DBa+, where D is the intrinsic(self-)diffusion coeffi- regions correspond to spinel. The presence of these two specific cient; the subscript represents the ionic species, and the superscript product phases confirms that the stabilized celsian contains finite represents the phase). Therefore, the cationic exchange between amounts of excess alumina; thus, the interfacial reaction is driven the Ba2+ species from the fluoromica and the Sr+ species from by the depletion and subsequent combination of Mg2+ cations the stabilized celsian is not a dominant diffusional mechanism and from the mica to form spinel, as described in Eq. (2). At reaction the interfacial is addressed almost exclusively via the temperatures of <1280C, the alumina-bearing stabilized-celsian/ upled interdiffusion of K+ and Si+ for Ba2+ and AP+. This fluorophlogopite interface remained stable-no breakdown of the particular exchange vector has been reported to occur in the mica because of reaction with the alumina was observed, which is celsian-adularia(KAISi,Os)feldspar series, -20 as well as in the consistent with the equilibrium articulated in Eq (2) atural kinoshitalite-phlogopite mica series.--- Both the The influence of the anisotropic fluoromica structure on the feldspars (BaI-KJAl os) and the micas interdiffusion process again is evidenced clearly along the stabi- (Ba,KI- Mg3[All+ Si3_,o(OH))exhibit complete isomor- lized-celsian/fluorophlogopite interface. The bicontrasted grain phous solid solution over their entire compositional ranges (i.e highlighted by the arrow in Fig. 4(a)illustrates the ease of where 0 s x s 1). Therefore, it is not surprising that diffusion along fluoromica basal planes than in th heterogeneous solid-solution equilibrium exists between stabilized direction. The brighter contrast entering from the A schematic representation of this layer edges of the fluoromica represent areas where the Ba+ Fig. 3(a) cation has been successfully exchanged for the k cation. The darkBa21 concentration correspond to a fluoromica grain that was aligned parallel to the interface), the coupled nature of this process is represented clearly by the simultaneous diffusion of K1 and Si41 cations into the fluoromica and of Ba21 and Al31 cations into the feldspar. Charge neutrality of the interdiffusing species is maintained, despite the required “uphill diffusion” (i.e., diffusion up a concentration gradient) of the Al31 species. Interdiffusion extended out as far as ;100 mm away from the interface in the mica and ;50 mm in the feldspar at this temperature, which is consistent with the information available from BEI (Fig. 2). Furthermore, the coupled cationic exchange between these two materials is isomorphous; i.e., both phases are stable against each other, but their compositions change. As illustrated in Fig. 3(b), the compositional profiles of both F2 and Mg21 cations remain constant throughout the fluoromica at levels consistent with bulk fluorokinoshitalite compositions, but then decrease abruptly to zero at the original stabilized-celsian/fluorokinoshitalite interface, demonstrating the phase stability of the fluoromica. A sharp change in Sr21 concentration at the interface also is evident in Fig. 3(b), which suggests that the mobility of Sr21 cations is much slower than that of Ba21 cations in the mica (i.e., DSr21 mica ,, DBa21 mica , where D is the intrinsic (self-)diffusion coeffi￾cient; the subscript represents the ionic species, and the superscript represents the phase). Therefore, the cationic exchange between the Ba21 species from the fluoromica and the Sr21 species from the stabilized celsian is not a dominant diffusional mechanism and the interfacial response is addressed almost exclusively via the coupled interdiffusion of K1 and Si41 for Ba21 and Al31. This particular exchange vector has been reported to occur in the celsian–adularia (KAlSi3O8) feldspar series,17–20 as well as in the natural kinoshitalite–phlogopite mica series.21–23 Both the feldspars ([Ba12xKx]Al22xSi21xO8) and the micas ([BaxK12x]Mg3[Al11xSi32x]O10(OH)2) exhibit complete isomor￾phous solid solution over their entire compositional ranges (i.e., where 0 # x # 1). Therefore, it is not surprising that a heterogeneous solid-solution equilibrium exists between stabilized celsian and fluorokinoshitalite. A schematic representation of this diffusion process is shown in Fig. 3(a). The interfacial compositions of the stabilized celsian and fluorokinoshitalite establish the thermochemical conditions for one of perhaps several two-phase equilibria. Affected by the use of specific starting compositions and the diffusion dynamics, many equilibrium tie lines could exist between a Ba-K solid-solution feldspar and a Ba-K solid-solution mica. Tables I and II display the experimentally measured interfacial compositions of all atomic species in the system at 1300°C (all compositional analysis was conducted at interfacial regions that showed minimal effects of mica anisotropy). Experimentally measured bulk compositions also are presented for comparison. Although the response observed at 1200°C was much less dramatic (interdiffusion of K1 and Si41 for Ba21 and Al31 extended away from the original interface only ;40 mm into the mica and ;25 mm into the feldspar), the interfacial compositions, shown in Tables III and IV, are quite similar to those measured at 1300°C. The apparent insensitivity of the interfacial compositions to the processing temperature suggests that thermochemical equilibrium can be established over a wide temperature range with a narrow range of compositions, despite the non-unique compositional conditions that generally describe a two-phase solid-solution equilibrium.24,25 Knowledge of these thermochemical conditions introduces the opportunity for the design of globally stable fluoromica interphase/silicate matrix interfaces. K(6Sr)-stabilized celsian and fluorokinoshitalite are not stable against each other at 1400°C. Fine particles of stoichiometric celsian (BaAl2Si2O8) and spinel (MgAl2O4) were the only addi￾tional phases observed along the interface. In addition, large (;2 mm) bubbles were produced along the exterior edges of the fluoromica substrate, indicating the volatilization of gaseous flu￾orine (F2). The formation of celsian and spinel phases, as well as gas bubbles, strongly suggests that interfacial reaction occurred via the combination of MgO from the fluoromica with slight alumina excesses from the feldspar-based matrix, as articulated by Eqs. (3) and (4). These results again suggest that MgO buffering may be highly effective in achieving thermodynamic equilibrium between alumina-rich stabilized celsian and fluorokinoshitalite, even at 1400°C. The thermochemical response between stabilized celsian and fluorophlogopite also was explored at elevated temperatures. Figure 4 shows micrographs of such an interface that has been processed at 1300°C. The gradual change in contrast within the fluorophlogopite, visible in the BEI image in Fig. 4(a), again indicates compositional gradients across the interface; however, in this case, high-Z elements increase in concentration toward the interface. EDS linescans (Fig. 5) confirm the interdiffusion of the K1 species from the mica and the Ba21 species from the feldspar (the coupled counterparts, Si41 exchange for Al31, although not shown in this figure, also were observed). Similar to the reaction between stabilized celsian and fluorokinoshitalite, uphill diffusion is observed again, except this time for the Si41 species. On closer examination of the 1300°C reaction, fine particles (,5 mm) of stoichiometric spinel (MgAl2O4) and leucite (K[AlSi2]O6) were observed along the interface via WDS analysis. Figure 4(b) shows a SEI image of the reaction, where the dark regions correspond to spinel. The presence of these two specific product phases confirms that the stabilized celsian contains finite amounts of excess alumina; thus, the interfacial reaction is driven by the depletion and subsequent combination of Mg21 cations from the mica to form spinel, as described in Eq. (2). At reaction temperatures of ,1280°C, the alumina-bearing stabilized-celsian/ fluorophlogopite interface remained stable—no breakdown of the mica because of reaction with the alumina was observed, which is consistent with the equilibrium articulated in Eq. (2). The influence of the anisotropic fluoromica structure on the interdiffusion process again is evidenced clearly along the stabi￾lized-celsian/fluorophlogopite interface. The bicontrasted grain highlighted by the arrow in Fig. 4(a) illustrates the relative ease of diffusion along fluoromica basal planes than in the perpendicular direction. The brighter contrast entering from the exposed inter￾layer edges of the fluoromica grain represent areas where the Ba21 cation has been successfully exchanged for the K1 cation. The dark Fig. 2. BEI micrograph of a stabilized celsian/fluorokinoshitalite reaction interface processed at 1300°C. The fluorokinoshitalite substrate separated from the stabilized celsian substrate on final polishing. Gradual contrast changes in both phases (the brighter phase is celsian, and the less-bright phase is mica) near the interface indicates an interdiffusion process of K1 replacing Ba21. Fluorokinoshitalite grains oriented with basal planes parallel to the interface show exceptionally bright contrast, in comparison to grains oriented perpendicular to the interface, indicating that these former grains do not participate easily in the ion exchange. September 2000 Thermochemical Reactions and Equilibria between Fluoromicas and Silicate Matrices 2291