A petrominetic (geological-analog) approach is applied to the design of alumina-fiber-reinforced glass-ceramic-matrix composites that use a refractory, trioctahedral fluoromica fiber-matrix interphase and feldspar matrixes. Studies of the solid-state reaction couples between these silicate phases are pursued to address the chemical tailorability of the interphase/matrix interface from an engineering perspective, The minimization of alumina and silica activities within polyphase, feldspar-based matrixes via MgO buffering is shown to be an effective route toward a stable fluoromica interphase. An anorthite-2-vol%-alumina (CaAl2Si2O8 + alpha-Al2O3) substrate, chemically buffered with MgO, is shown to exhibit thermodynamic stability against fluorokinoshitalite (BaMg3[Al2Si2]O10F2), up to temperatures potentially as high as 1460 degrees C. The keg to the approach is the reduction of alumina activity via the formation of MgAl2O4 spinel. Similarly, the formation of forsterite (Mg2SiO4) stabilizes the mica in contact with matrix compositions otherwise containing excess silica. The cationic interdiffusion between solid-solution feldspars and fluoromicas also is characterized. Coupled interdiffusion of K+ and Si4+ in exchange for Ba2+ and APC was observed between K-Ba solid-solution celsian and the barium-rich solid-solution end-member fluorokinoshitalite at 1300 degrees C. A similar cationic exchange also is observed against the potassium-rich end-member fluorophlogopite (KMg3[AlSi3]O10F2), although in a reverse direction, at temperatures of <1280 degrees C, The interfacial compositions identified via electron microprobe analysis specify one set of local equilibrium conditions from which global ceramic composite equilibrium can be achieved.