New insights into Mn²⁺ and Mg²⁺ inhibition of calcite growth

Journal Article · Mon Jun 20 00:00:00 EDT 2022 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2022.06.015· OSTI ID:1994355

Mills, Jennifer V. ^[1]; Barnhart, Holly A.  ^[2]; DePaolo, Donald J. ^[3]; Lammers, Laura N.  ^[3]

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1. Univ. of California, Berkeley, CA (United States)
2. California Institute of Technology (CalTech), Pasadena, CA (United States)
3. Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)

Impurity ion and isotope partitioning into carbonate minerals provide a window into the molecular processes occurring at the fluid-mineral interface during crystal growth. Here, we employ calcium isotope fractionation together with process-based modeling to elucidate the mechanisms by which two divalent cations with starkly contrasting compatibility, magnesium and manganese, inhibit calcite growth and incorporate into the mineral lattice. Calcite growth inhibition by Mg²⁺ is log-linear and K_Mg is on the order of 0.02–0.03 throughout the range of {Mg²⁺}/{Ca²⁺} studied here (0.01–2.6). Mn²⁺ exhibits much stronger log-linear growth rate inhibition at low Mn²⁺ concentrations (fluid {Mn²⁺}/{Ca²⁺} = 0.001–0.02). Mn²⁺ is readily incorporated into the calcite lattice to form a calcite-rhodochrosite solid solution, with large partition coefficients (K_Mn 4.6–15.6) inversely correlated to growth rate. For both Mn²⁺ and Mg²⁺, calcium isotope fractionation is found to be invariant with {Me²⁺}/{Ca²⁺} despite more than an order of magnitude decline in growth rate. This invariant Δ^44/40Ca suggests that the presence of Mn²+ or Mg²⁺ does not significantly change the relative rates of Ca²⁺ attachment and detachment at kink sites during growth, indicative of a dominantly kink blocking inhibition mechanism. Because the partitioning behavior dictates that Mn²⁺ must attach to the surface significantly faster than Ca²⁺, attachment of Mn²⁺ is likely to be as a non-monomer species such as an ion pair or possibly a larger polynuclear cluster. We propose that calcite growth rate inhibition by Mn is determined by the kinetics of carbonate attachment at Mn-occupied kink sites, potentially due to slow re-orientation kinetics of carbonate ions that have formed an inner-sphere complex with Mn²⁺ at the surface but must reorient to incorporate into the lattice. We demonstrate that patterns in Mg²⁺ partitioning and inhibition behavior are broadly consistent with growth inhibition driven by slow Mg²⁺-aquo complex dehydration relative to Ca²⁺ but argue that this mechanism likely represents one endmember scenario, seen in Mg-calcite growth at low supersaturations and net precipitation rates. During growth at faster net precipitation rates, some portion of Mg²⁺ is likely incorporated as a partially hydrated or otherwise complexed species, but calcite growth remains significantly inhibited by the kinetics of CO₃^2- attachment at Mg²⁺ kink sites. These findings suggest a hybrid classical/nonclassical growth mechanism whereby Ca²⁺ incorporates largely as a free ion at kink sites while Mn²⁺ and some portion of Mg²⁺ are incorporated via non-monomer attachment. This pattern may be generalizable; trace constituent cations with aquo-complex desolvation rates significantly slower than the mineral growth rate preferentially incorporate as a non-monomer species during otherwise classical crystal growth.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division (CSGB)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1994355
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 334; ISSN 0016-7037
• Publisher: Elsevier; The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
magnesium
manganese
calcite
calcium isotope fractionation
kinetic isotope effect
trace element partitioning
nonclassical growth