Special Vaterite Found in Freshwater Lackluster Pearls

In recent years, scientists have special interests in biovaterite found in freshwater cultured lackluster pearls in south China. Vaterite is an unstable crystalline form of calcium carbonate and rare in nature. In this work, we first reported that vaterite crystals in freshwater lackluster pearls have a similar hiberarchy to that of nacre with the “brick and mortar” structure. The dimension of a single vaterite tablet is approximate 8 × 2 × 0.4 μm³. In half-lackluster pearls, the aragonite−vaterite interface is abrupt, and the boundary is sequential; two different kinds of contact modes between aragonite and vaterite tablets were observed. Furthermore, crystallographic orientation regulation of vaterite tablets was analyzed for the first time. The results show that vaterite tablets are arrayed with the [010] direction perpendicular to the tablet plane; strong [010] texture and relatively weak [101] and [102] texture were obtained in a region from several neighboring tablets to micrometer scale. In addition, the misorientation angles between adjacent tablets mainly concentrate on small angles lower than 10° and have another gather in 60°.