Chemical controls on the magnesium content of amorphous calcium carbonate

Abstract

Amorphous calcium carbonate (ACC) is a metastable phase that forms in diverse biogeochemical settings. This material can incorporate significant amounts of magnesium and other elements, but the conditions that regulate composition are not established. Using a mixed flow reactor method, we synthesize Mg-free ACC (control) and amorphous magnesium calcium carbonate (ACMC) under controlled chemical conditions to determine the relationship between composition and inorganic solution chemistry. Input solutions contained a constant initial Mg/Ca ratio of 5/1 with variable total carbonate concentration, pH, and supersaturation. Within the reactor, input solution chemistry evolves in proportion to the extent of precipitation whereby the initial Mg/Ca ratio increases to values as high as 14 at steady state conditions. By this approach, we produce reproducible quantities of ACMC with 24 to >70 mole % Mg to give compositions of Mg (0.24-0.72) Ca (0.76-0.28) CO 3 1.42-1.63 H 2 O. The primary control on ACMC composition is the steady state solution composition that develops in the reactor during precipitation. Analysis of the data shows the Mg content of ACMC is regulated by the interplay of three factors at steady state conditions: 1) Mg/Ca ratio; 2) total carbonate concentration; and 3) solution pH. Using the Henderson-Kracek model to estimate the partition coefficients for the Mg content of ACMC, we find K D is approximately constant at 0.047± 0.003 when steady state pH is less than 9.5, but values of K D triple as steady state pH increases from 9.5 to 10.3. Our K D values are lower than previous estimates that are based upon initial solution composition. In contrast, our estimates of K D are determined from the solution chemistry at steady state conditions and for pH conditions that are less extreme than previous experimental studies. We suggest the approach of using steady state composition to estimate K D gives a more accurate representation of relationships between ACMC composition and local conditions. The findings demonstrate local pH and total carbonate concentration can be regulated at the time of formation to produce Mg amorphous carbonates of desired composition.

Composition Systematics in the Exoskeleton of the American Lobster, Homarus americanus and Implications for Malacostraca

“…First, studies show that increasing concentrations of Mg and P increase the lifetime of the metastable amorphous polymorph optimize proportions of ACC to calcite for the duration of the molt cycle. Also, in vitro studies show Mg and P also progressively modify ACC morphology (Xu et al, 2005;Blue and Dove, 2015). In contrast to the more uniform size and shape of impurityfree ACC particles, incorporation of Mg or P produces more variably sized aggregated spheroids (Xu et al, 2005;Blue and Dove, 2015).…”

Sedimentological perspectives on climatic, atmospheric and environmental change in the Neoproterozoic Era

“…An important point is that relatively little is known about the trace element contents of these phases, except that they may be enriched because of relatively poor discrimination of ions during rapid growth (e.g. Blue & Dove, ). This could mean, for example, that the assumption that Sr‐rich limestones were originally aragonitic may not always be correct.…”

High‐Magnesium Calcite Mesocrystals: Formation in Aqueous Solution under Ambient Conditions

“…[21] Attempts have also been made to prepare high-Mg calcites in aqueous solutions without organic additives.T he methods were to control the phase transformation of Mg-ACC by limiting the contact between water and Mg-ACCi naconfined space, [22] or by the experimental parameters such as temperature,pH, and the activities of the cations and the carbonate ions. [23][24][25][26][27] While these studies in aqueous solutions have deepened our understanding of the thermodynamic controls on the phase transformation of calcium carbonate,t he biological relevance of these studies awaits further substantiation because biominerals usually have very unique hierarchical levels of structure formation. [28] In particular,t he notion of mesocrystal has been coined to describe the fascinating formation of a" single crystal" through the aggregation of nanocrystals.…”