Stabilities of crystal faces of aragonite-type strontianite (SrCO3) and cerussite (PbCO3) compared by AFM observation of facet formation in acid

Micro-facet formation processes were observed, with atomic force microscopy (AFM), at various crystal faces of two aragonite-type minerals, cerussite (PbCO3) and strontianite (SrCO3), in dilute aqueous acetic acid. Among the prismatic faces, parallel to the c-axis, electrically neutral (1 0 0) and (1 3 0) faces were stabilized in the acidic environment, as in the case of aragonite (CaCO3). On the other hand, polar (1 1 0) and (0 1 0) faces of PbCO3 remained to be stable, in contrast with the case of aragonite. Even atom-resolved AFM images were observed with the (1 1 0) face, corresponding to the arrangement of carbonate ions, most probably protonated. Since Pb2+ is a more polarizable soft acid, Pb–O bonds bear covalent nature in addition to Coulombic nature. Due to diminished polarity with the bonds, the (1 1 0) face remained stable even in the acidic condition. Among the crystal faces intersecting with the c-axis, stabilization, in the acidic condition, of (1 1 1) faces of SrCO3 and (0 2 1) faces of PbCO3 was observed in contrast to the stabilization of (1 1 2) faces of aragonite, suggesting flexible nature of the mineral faces having larger cations. Relationship between natural crystal forms and growth environment was discussed.