The ultrastructure of pyroxenoid chain silicates. III. Intersecting defects in a synthetic iron-manganese pyroxenoid

A synthetic pyroxenoid of nominal composition MnFeSi₂O₆ has been produced in the glassy state and examined by high-resolution electron microscopy after periods of annealing of from two to sixteen hours at 1070 K. Initially highly defective structures exhibiting large chain repeat distances are produced, but on further annealing the five-tetrahedra chain repeat of rhodonite predominates. (110)-type stacking faults are frequently observed. In most eases these faults involve considerable disruption of the lattice, but in certain instances the stacking fault displacement is equal to the rhodonite lattice repeat and the faults are visible only if the chain periodicity varies. In the latter case, the structure shows a two-dimensional ability to accommodate defects, and models for this, involving chain breaking and/or chain branching, are proposed.