Additional reflections and polytypic sequences in Polygonal Serpentine

The existence of polygonal serpentine was first noticed trough additional reflections in XRPD patterns. Later, HRTEM revealed micrometric cross-sections, consisting of polygonally arranged lizardite sectors. They occur rotated by 12° or 24°, originating the “magic” numbers of 30 and 15 sectors per fiber (PS-30 and PS-15). Adjacent sectors are based on different, regularly repeated polytypic stacking sequences. The stacking vector between adjacent layers changes by +/- b/3 (equivalent to -/+ b/6) from one sector to the next. Moving counterclockwise, couple of layers are stacked by (o) orthogonal, (r) right or (l) left pointing vectors in PS-30, and by (o), (l) and (r) pointing vectors in PS-15. The two sequences arise because adjacent sectors have (001) rotated by 24° and 12° in PS-15 and PS30, respectively; therefore, the PS-15 olr sequence arises eliminating bracketed sectors in the o(r)l(o)r(l)o(r)l.. sequence of PS-30. Twolayer and multilayer polygonal polytipic arrangements are common, still matching the same rules as one-layer arrangements. HRTEM observations indicate continuous 1:1 layer, with no tetrahedral inversion between adjacent sectors. The complex [100] SAED patterns with five-fold symmetry are reproduced by properly overlapped hk0 reciprocal lattice planes. The contemporaneous presence of different unit-cells removes degeneracy in dhkl values of adjacent sectors, thus leading to clusters of additional reflections (e.g., 020 and 020r,l; 021r, 021o and 021l; 022r, 022o and 022l in PS-30) in the XRPD patterns.