Mg-cordierite: Si/Al ordering,optical properties,and distortion

Flux-grown crystals of Mg-cordierite, Mg_1.93 Al_3.95 Si_5.07 O₁₈ synthesized by Lee and Pentecost (1976) appear biaxial (2V _x=10°–25°) under the polarizing microscope whereas their distortion index =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, 2V _x increased. Simultaneously, undulatory extinction and intragrain variations in 2V _x increased slightly up to 4 h annealing, then steadily decreased. For this Mg-cordierite, which lacks significant channel H₂O or CO₂, 2V _x and reach maxima of 88° (=589 nm) and 0.25° after 42 h of annealing but other sectors still display lesser values for 2V _x. Presumably, to the extent 2V _x 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.The cause of intermediate values for , whether compositional or from submicroscopic {110} or {310} twinning, may be revealed by single crystal X-ray photographs. Streaking of diffraction spots along a^* or b^* (but not c^*) will indicate such twinning as the sole or major cause.