Experimental coarsening of antiphase domains in a silicate mineral.

Combined annealing experiments and observations by transmission electron microscopy show that in natural pigeonite crystals antiphase domains coarsen approximately according to a rate law in which the tenth power of the average domain size is proportional to time. This result suggests that certain cations (possibly Ca(2+)) were segregated preferentially onto the antiphase boundaries. The domain size in samples quenched from above the high-low transformation temperature is large and apparently independent of annealing time and temperature. It appears that large domains can be generated either by very fast or by very slow cooling; thus the estimation of geological cooling rates from the sizes of antiphase domains in natural samples becomes rather difficult.