yournal of the American Ceramic Sociery-King et al. (a) than either fluorokinoshitalite or anorthite-2 vol%o alumina sub- sequent WDS analysis of this high-Z layer revealed a composition that corresponded to stoichiometric celsian(i.e, BaAl,Si,Os). At fluorokinoshitalite 1286 C. the reaction intensified and resulted in a wider reaction zone (-60 Hm) that consisted of stoichiometric celsian, spinel, and anorthite(Fig. 1(b). The presence of spinel confirms that the reaction at the interface is driven by the combination of MgO from fluorokinoshitalite and excess Al2O, from the anorthite pellet, as shown in the following two-step reaction BaMg.[Al SigJO1oF2+5O2(g)- BaAl2Si2O8 anorthite +2 vol% alumina 3Mgo+ f, (g) MgO+Al2O3→MgAl2O4 4) Fluorokinoshitalite is not in thermodynamic equilibrium with anorthite-2 vol% alumina at 1200%C In an attempt to minimize the destruction of the fluoromica phase via the depletion of Mg cations, a MgO-buffered, (b)2 anorthite-2-vol%alumina pellet (now alumina +2 vol% spinel) fluorokinoshitalite was reacted against fluorokinoshitalite at 1200%C. The interfacial illustrated Fig. I(c). Although the fluorokinoshitalite/MgO-buffered anorthite interface was still slightly reactive(governed by eqs. (3)and (4), the reaction was significantly reduced (compare Fig. I(a)), as illustrated by the now-thinner (-2 Hm thick), discontinuous celsian layer. The arrows highlight regions where interfacial celsian formation oc- curred. The generation of smaller quantities of product phases (i.e celsian) clearly establishes the effectiveness of Mgo buffering in omoting stable trioctahedral fluoromica/anorthite +2 vol% umina interfaces by arresting the loss of Mgt cations from the phyllosilicate. Complete minimization of the interfacial reaction in this case could be accomplished by increasing the amount of Mgo anorthite+2 vol% alumina additions to the anorthite-2-vol%-alumina matrix and/or increas- ing the high-temperature annealing time of the buffered feldspar 48 HIRONS let, to ensure that all free alumina has reacted to spinel. These results also are applicable to silica-rich (i.e, anorthite + minor hase-pure silica) matrix compositions: for Mgo buffering in this case, the silica is combined with the mgo to form forsterite thereby substantially diminishing the silica activity in the matrix. Combining the ideas, it is important to note that the stoichiometric Mgo-buffered anorthite MgAl, O, is in equilibrium with forsterite: thus, a two-phase pinel forsterite buffer within the feldspar matrix should stabilize the Mg+-trioctahedral mica completely (2) Cationic Interdiffusion: Equilibrium between Multicomponent Feldspar and Fluoromica Figure 2 shows a BEl image of a stabilized-celsian/fluoroki- noshitalite interface processed at 1300C. Although the reaction couple split along the interface during the final stages of polishing, fluorkkinoshitalite as evidenced by the -20-pm-wide interfacial crack, the separated bstrates still illustrate the entire thermochemical response at 1300%C. One can observe that the contrast in the mica darkens ithin -100 um of the interface, whereas that in the feldspar brighter in the first -50 um beyond the interface. In general, the result indicates an increase in the concentration of high-z elements Fig. 1. B El micrographs illustrating the morphology of reaction interfaces into the celsian and their loss from the mica. The only exceptions to this trend are large, more brightly contrasted fluorokinoshitalite 2 vol% alumina/fluorokinoshitalite reaction at 1200C (the primary grains that are oriented with their basal planes parallel to the reaction-induced feature consists of a single, continuous layer of stoichiometric reaction-couple interface, clearly demonstrating the impact of the sian, 6 um thick),(b) anisotropic fluoromica crystal structure on the interfacial response. on at 1286.C(the reaction in Fig. 1(a) intensifies, producing product These observed compositional trends across the interface are metric celsian, anorthite, and spinel), and (c) Mgo-buftered indicative of an interdiffusional process between the feldspar and ite/filuorokinoshitalite reaction at 1200C(notice the les-severe, discon- the fluoromica. Electron microprobe analysis of a bonded section nature of the celsian layer, highlighted by the arrows, in comparison he interface without Mgo additions(see Fig. 1(a))) of the same sample identifies the diffusing ionic species as K Si, Ba, and Al, which are displayed in a WDS concentra- tion profile across the interface(see Fig. 3(a)). Although the diffusional anisotropy of the fluoromica results in considerable alumina. The brightest feature in Fig. 1(a) is a continuous, variation in the X-ray signals (particularly in the 5 um section "6-um-thick interfacial layer that corresponds to a phase com- ent to the original interface on the fluorokinoshitalite side, the posed of a relatively higher concentration of heavy- elements than-expected K+ concentration and higher-than-expectedalumina. The brightest feature in Fig. 1(a) is a continuous, ;6-mm-thick interfacial layer that corresponds to a phase com￾posed of a relatively higher concentration of heavy-Z elements than either fluorokinoshitalite or anorthite–2 vol% alumina. Sub￾sequent WDS analysis of this high-Z layer revealed a composition that corresponded to stoichiometric celsian (i.e., BaAl2Si2O8). At 1286°C, the reaction intensified and resulted in a wider reaction zone (;60 mm) that consisted of stoichiometric celsian, spinel, and anorthite (Fig. 1(b)). The presence of spinel confirms that the reaction at the interface is driven by the combination of MgO from fluorokinoshitalite and excess Al2O3 from the anorthite pellet, as shown in the following two-step reaction: BaMg3@Al2Si2#O10F2 1 1 2 O2~ g! 3 BaAl2Si2O8 1 3MgO 1 F2~ g! (3) MgO 1 Al2O3 3 MgAl2O4 (4) Fluorokinoshitalite is not in thermodynamic equilibrium with anorthite–2 vol% alumina at 1200°C. In an attempt to minimize the destruction of the fluoromica phase via the depletion of Mg21 cations, a MgO-buffered, anorthite–2-vol%-alumina pellet (now alumina 1 2 vol% spinel) was reacted against fluorokinoshitalite at 1200°C. The interfacial response is illustrated as a BEI image in Fig. 1(c). Although the fluorokinoshitalite/MgO-buffered anorthite interface was still slightly reactive (governed by Eqs. (3) and (4)), the reaction was significantly reduced (compare Fig. 1(a)), as illustrated by the now-thinner (;2 mm thick), discontinuous celsian layer. The arrows highlight regions where interfacial celsian formation oc￾curred. The generation of smaller quantities of product phases (i.e., celsian) clearly establishes the effectiveness of MgO buffering in promoting stable trioctahedral fluoromica/anorthite 1 2 vol% alumina interfaces by arresting the loss of Mg21 cations from the phyllosilicate. Complete minimization of the interfacial reaction in this case could be accomplished by increasing the amount of MgO additions to the anorthite–2-vol%-alumina matrix and/or increas￾ing the high-temperature annealing time of the buffered feldspar pellet, to ensure that all free alumina has reacted to spinel. These results also are applicable to silica-rich (i.e., anorthite 1 minor phase-pure silica) matrix compositions: for MgO buffering in this case, the silica is combined with the MgO to form forsterite, thereby substantially diminishing the silica activity in the matrix. Combining the ideas, it is important to note that the stoichiometric MgAl2O4 is in equilibrium with forsterite:16 thus, a two-phase spinel 1 forsterite buffer within the feldspar matrix should stabilize the Mg21-trioctahedral mica completely. (2) Cationic Interdiffusion: Equilibrium between Multicomponent Feldspar and Fluoromica Figure 2 shows a BEI image of a stabilized-celsian/fluoroki￾noshitalite interface processed at 1300°C. Although the reaction couple split along the interface during the final stages of polishing, as evidenced by the ;20-mm-wide interfacial crack, the separated substrates still illustrate the entire thermochemical response at 1300°C. One can observe that the contrast in the mica darkens within ;100 mm of the interface, whereas that in the feldspar is brighter in the first ;50 mm beyond the interface. In general, the result indicates an increase in the concentration of high-Z elements into the celsian and their loss from the mica. The only exceptions to this trend are large, more brightly contrasted fluorokinoshitalite grains that are oriented with their basal planes parallel to the reaction-couple interface, clearly demonstrating the impact of the anisotropic fluoromica crystal structure on the interfacial response. These observed compositional trends across the interface are indicative of an interdiffusional process between the feldspar and the fluoromica. Electron microprobe analysis of a bonded section of the same sample identifies the diffusing ionic species as K1, Si41, Ba21, and Al31, which are displayed in a WDS concentra￾tion profile across the interface (see Fig. 3(a)). Although the diffusional anisotropy of the fluoromica results in considerable variation in the X-ray signals (particularly in the 5 mm section adjacent to the original interface on the fluorokinoshitalite side; the lower-than-expected K1 concentration and higher-than-expected Fig. 1. B EI micrographs illustrating the morphology of reaction interfaces between fluorokinoshitalite and anorthite-based glass-ceramics ((a) anorthite 1 2 vol% alumina/fluorokinoshitalite reaction at 1200°C (the primary reaction-induced feature consists of a single, continuous layer of stoichiometric celsian, 6 mm thick), (b) anorthite 1 2 vol% alumina/fluorokinoshitalite reaction at 1286°C (the reaction in Fig. 1(a) intensifies, producing product phases of stoichiometric celsian, anorthite, and spinel), and (c) MgO-buffered anorthite/fluorokinoshitalite reaction at 1200°C (notice the less-severe, discon￾tinuous nature of the celsian layer, highlighted by the arrows, in comparison with the interface without MgO additions (see Fig. 1(a))). 2290 Journal of the American Ceramic Society—King et al. Vol. 83, No. 9