X-ray absorption spectroscopy study of Cu²⁺ and Zn²⁺ adsorption complexes at the calcite surface: Implications for site-specific metal incorporation preferences during calcite crystal growth

We report results from in situ extended X-ray absorption fine structure (EXAFS) spectroscopy studies of Cu(II) and Zn(II) complexes forming at the calcite surface following adsorption from preequilibrated calcite-saturated solutions. Both Cu(II) and Zn(II) coordinate at Ca sites on the calcite surface, forming mononuclear inner-sphere adsorption complexes. The Zn adsorption complexes are in tetrahedral coordination with first-shell O neighbors with R_Zn-O = 1.95 Å, and the Cu complexes are Jahn-Teller distorted, with equatorial R_Cu-O = 1.95 Å. Results from EXAFS data of dilute Cu- and Zn-calcite solid solutions confirm substitution of these metals in the Ca site of the calcite structure as octahedral complexes during coprecipitation. X-ray fluorescence microanalyses of calcite (101̄4) hillocks grown in coprecipitation experiments show that divalent Cu and Zn, which have ionic radii smaller than Ca, are preferentially incorporated into the parallel arrays of <4̄41>₊ steps that define one pair of symmetrically equivalent vicinal faces on polygonized growth spirals. In contrast, other divalent metals with sixfold ionic radii smaller than Ca (Co, Cd, Mn, Mg) have been shown to be preferentially incorporated into <4̄41>_- growth steps, which define the second pair of vicinal faces on the growth spirals, but which are symmetrically nonequivalent to the steps on the first pair. The distortion from octahedral symmetry observed for the Cu and Zn adsorption complexes likely plays a key role in the observed preference of Cu and Zn for incorporation into the <4̄41>₊ steps.