This study concerns the bulk mica phase of a barium phlogopite glass-ceramic, with potential applications in Computer Aided Design and Computer Aided Manufacturing in dentistry, which has been studied in detail and characterized for the first time. A number of analytical techniques including powder X-ray diffraction (XRD), scanning electron microscopy, energy dispersive spectroscopy, electron micro-probe analysis (EMPA), single crystal XRD and Monte-Carlo methods have been used to determine the mica phase composition and crystallographic structure. This has led to the identification of a new species of trioctahedral interlayer deficient brittle mica with an ideal formula of Ba2/3Mg3(Si8/3Al4/3)O10F2. Monte-Carlo simulations of Si/Al cation ordering indicate that the (Si8/3Al4/3) tetrahedral composition is unique and energetically favoured over that of the original assumed mica phase of Ba0.5Mg3(Si3AlO10)F-2. The general mica composition X0.5Mg3(Si3Al)O10F2 where X is a divalent interlayer cation; therefore, does not precipitate in brittle mica glass-ceramics.