Flux-grown crystals of Mg-cordierite, Mg1.93 Al3.95 Si5.07 O18 synthesized by Lee and Pentecost (1976) appear biaxial (2Vx=10°–25°) under the polarizing microscope whereas their distortion index Δ=0°. Between crossed polars, (001) sections display lamellar and cyclic twinning on {110} and, less frequently, {310}. As duration of annealing at 1,300° C, increased, 2Vx increased. Simultaneously, undulatory extinction and intragrain variations in 2Vx increased slightly up to 4 h annealing, then steadily decreased. For this Mg-cordierite, which lacks significant channel H2O or CO2, 2Vx and Δ reach maxima of 88° (λ=589 nm) and 0.25° after 42 h of annealing but other sectors still display lesser values for 2Vx. Presumably, to the extent 2Vx is less than 88°, these sectors represent intergrown submicros copically twinned orthorhombic domains and thus possess shortrange but not long-range order. Annealing at 1,300° C likely increased long-range order by promoting growth of larger domains at the expense of smaller ones. Ultimately, two differently oriented domains, growing toward each other by annexation (and re-orientation) of smaller domains, meet in a twin boundary that, with time, tends to become straight.