The morphology of calcite crystals grown in a porous medium doped with divalent cations

Calcite crystals were grown in the presence of small concentrations (50, 200, and 600 ppm) of divalent cations (Ba²⁺, Sr²⁺, Co²⁺ and Mn²⁺) in a silica hydrogel medium. The calcite crystals grown in the presence of cations larger than Ca²⁺ (Ba²⁺ or Sr²⁺) developed rhombohedral habits defined by {101¯4} form, similar to the morphology of calcite grown in a pure gel. SEM images show that growth on {101¯4} occurs by lateral advancement of layers bounded by macroscopic dendritic or jagged steps. In the case of calcite crystals grown in a gel doped with cations smaller than Ca²⁺ (Co²⁺ or Mn²⁺), a variety of morphologies was obtained, ranging from blocky crystals (at lower concentrations: 50 and 200 ppm) to peanut-like aggregates, spheres and spherulites (at 600 ppm). The macroscopic morphological characteristics of such doped calcite crystals reflect closely the growth behaviour of calcite {101¯4} surface at a nanoscale, reported by previous AFM studies. Morphological features have been interpreted on the basis of the modification of growing steps characteristics as a consequence of asymmetrical cation incorporation. The use of such morphologies as a criterion of biological activity is, therefore, unreliable.