Stacking faults and polytypism in opal, SiO₂.nH₂O

A replication electron microscopy study of etched fracture surfaces of opal revealed four distinctive patterns of packing of the constituent SiO₂ spheres. These patterns could be accounted for by consideration of appropriate stacking faults or modifications in a cubic closest packed structure. These were (1) intrinsic stacking faults (2) extrinsic stacking faults (3) twinning and (4) rhombohedral polytypism. The success in predicting some of the observed packing patterns by assuming stacking modifications of a cubic closest packed structure indicates the existence of a basic, though frequently faulted, f.c.c. opal structure. Screw dislocation-spiral growth is proposed as a possible mechanism for explaining the occurrence of the ordered opal structures.