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

Mills, Jennifer V. and Barnhart, Holly A. and DePaolo, Donald J. and Lammers, Laura N. (2022) New insights into Mn²⁺ and Mg²⁺ inhibition of calcite growth. Geochimica et Cosmochimica Acta, 334 . pp. 338-367. ISSN 0016-7037. doi:10.1016/j.gca.2022.06.015.

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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^(44/40)Ca 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 CO₃²⁻ 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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Mills, Jennifer V.0000-0003-1775-5509
Barnhart, Holly A.0000-0002-0842-9464
DePaolo, Donald J.0000-0001-8880-0202
Lammers, Laura N.0000-0002-8509-7571
Additional Information:© 2022 Published by Elsevier. Received 21 October 2021, Revised 7 June 2022, Accepted 9 June 2022, Available online 20 June 2022. This work was supported by a Berkeley Fellowship to J.V.M, and funding from the DOE Office of Science, Basic Energy Sciences, Division of Chemical, Biological and Geological Sciences under contract no. DE-AC02-05CH11231. The authors wish to thank Rolf Arvidson and two anonymous reviewers whose thoughtful comments improved this manuscript. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funding AgencyGrant Number
University of California, BerkeleyUNSPECIFIED
Department of Energy (DOE)DE-AC02-05CH11231
Subject Keywords:Magnesium; manganese; calcite; calcium isotope fractionation; kinetic isotope effect; trace element partitioning; nonclassical growth
Record Number:CaltechAUTHORS:20220705-346474000
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:115321
Deposited By: George Porter
Deposited On:08 Jul 2022 00:06
Last Modified:11 Nov 2022 22:54

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