electron microscopy (SEM and TEM). Pores emanating from surface pits propagate along the <111>A crystallographic directions to

form, in the early stages of anodization, porous domains with the shape of a tetrahedron truncated symmetrically through its center

by a plane parallel to the surface of the electrode. This was confirmed by comparing the predictions of a detailed model of pore

propagation with SEM and TEM observations. The model showed in detail how <111>A pore propagation leads to domains with the

shape of a tetrahedron truncated by a (100) plane. Observed cross sections corresponded in detail and with good precision to those

predicted by the model. SEM and TEM showed that cross sections were trapezoidal and triangular, respectively, in the two cleavage

planes of the wafer, and TEM showed that they were rectangular parallel to the surface plane, as predicted. Aspect ratios and angles

calculated from observed cross sections were in good agreement with predicted values. The pore patterns observed were also in good

agreement with those predicted and SEM observations of the surface further confirmed details of the model.